Tap Changing Current Regulator

Abstract

A constant current control system for lighting applications, in particular airport series lighting arrangements, using a multi-tap transformer connected to the load effectively through back-to-back thyristors which are selectively fired to connect the various taps to the load in order to regulate the current. A signal representative of the square of the current is used to produce the control signal which controls the firing point of the various thyristors. The control signal is a series of impulses produced by a multivibrator.

Description

BACKGROUND OF THE INVENTION

This invention relates to the regulation of power supplied to a load, in particular a series lighting load such as a lighting arrangement conventionally used for airport lighting.

In the prior control of airport lighting, it has been conventional to use saturable reactors to control the current supplied to the series-arranged lamps. It is conventional in the operation of such lamps that a short circuit be placed across the lamps when they are not operative. When the supply is applied to this load in its short circuit condition, it will be evident that, in the absence of regulation, prohibitively large currents would flow. However, in the prior art it was usual, as has been previously indicated, to unsaturate the reactor under these conditions and cause the short circuit to appear, so far as the actual source of energy is concerned, as a highly reactive load. The current was then limited in accordance with the reactance to the permissible current limit. The resultant power would be essentially wholly reactive and would represent the source voltage times the reactive current in VARS. Such a reactive load is obviously disadvantageous particularly in view of power factor penalties which are assessable by suppliers and the excessively high line currents when supplied by standby Diesel generator sets. It would therefore be desirable to reduce this if possible. Saturable reactors also had disadvantages in their slow response time, high losses and severe waveform distortion would could give rise to undesirable harmonics and radiation, which are particularly objectionable in the vicinity of an airport.

SUMMARY OF THE INVENTION

These and other objections are overcome in the present invention by utilizing solid state switching devices, not only to regulate the current by controlling the conducting angle of the solid state devices, but also selectively to supply current from different taps of a supply transformer in accordance with the current demand so that the wave shape is more closely related to sinusoidal, and also that in short circuit conditions a lower voltage is applied from the source, thus reducing the total VAR consumption of the system under these short circuit conditions.

A clearer understanding of our invention may be had from a consideration of the following drawings, in which:

FIG. 1 is a schematic diagram of a system in accordance with our invention; and

FIG. 2 is a waveform diagram useful in explaining the operation of the circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be seen in FIG. 1, the load transformer 10 is supplied from a source of 60-cycle alternating current at the available voltage as convenient. The secondary of this transformer is provided with several taps. Connected to one of these taps, designated 11, are a pair of thyristors 12 and 13 arranged back-to-back. The other ends of these thyristors are connected together and to the primary of the output transformer 14. A similar pair of thyristors designated 15 and 16 are connected to tap 17 and to the primary of output transformer 14. To the output of the output transformer 14 is connected the load, consisting of a series of lamps 18, 18a, 18b, etc., each supplied through its individual transformer 22, 22A, 22B, etc. This load is connected to the secondary of the transformer through a series of contacts 20a, 20b, 20c and 20d, all of which are interrelated so that when 20a and 20b are closed, 20d and 20c are open and conversely when 20d and 20c are closed, 20a and 20b are open.

A current transformer 19 is connected in series with the primary of transformer 14 and the output of this transformer is applied to transducer 21. Transducer 21 is arranged to produce a positive potential representative of the square of the current passing through the current transformer 19. This signal is passed through resistor 24 to the operational amplifier 28.

This operational amplifier, together with all similar operational amplifiers in the system, is provided with the usual feedback circuits and potentials to permit its normal operation, all of which are not shown. Also applied to the operational amplifier 28 is a reference signal derived from potentiometer 26 which is negative in polarity and is applied through resistor 27 to the input of the operational amplifier. This reference signal establishes the desired brilliance of the lamps 18. As will be seen, the input to the operational amplifier 28 is the sum of a positive and a negative signal, and therefore represents the difference between the output from transducer 21 and the reference, which may be referred to as an error. This error is integrated in amplifier 28 which is not a simple operational amplifier, but is an integrating amplifier. As a result, its output represents the accumulated error. This output is applied to a further operational amplifier 29, whose point of operation is established by the bias provided from potentiometer 30. When and only when the signal from the operational amplifier 28 exceeds the bias potential, amplifier 29 produces an output which is applied through resistor 33 to operational amplifier 31.

A further signal is applied to operational amplifier 31 from the ramp generator 32 which is provided with a reference frequency at 60 cycles, which corresponds to the source. In response to this reference, the ramp generator produces a linearly decreasing sawtooth wave, as shown at C in FIG. 2, in phase synchronism with the waveform of the source and having a duration of one-half cycle. This ramp is applied through resistor 34 to amplifier 31. Amplifier 31 is arranged so that it provides at its output as long as the combination of the interated signal and the ramp is negative, a positive output.

The next portion of the circuit consists of an operational amplifier arranged as a multivibrator, controlled by the potential at the junction of resistors 35 and 36. Operational amplifier 37 and its associated components operate as a multivibrator, producing a series of square waves at the output terminal which is connected to resistor 38. The mode of operation of the multivibrator will be explained in greater detail subsequently, but it will be noted that the output is connected to the input through resistors 39 and 40 and diodes 41 and 42. The input of the amplifier is also connected to ground through capacitor 43 and across the capacitor is connected a diode 44 and a Zener diode 45 in series. The other input of the amplifier, that is, the non-inverting input, is connected through resistor 48 to a potentiometer consisting of resistors 46 and 47 which are connected between the output of the amplifier and ground. This input terminal of the amplifier is also connected through resistor 49 and diode 77 to the junction of resistors 35 and 36. The square waves produced by the amplifier 37 functioning as a multivibrator are applied through resistor 38 to the pulse forming network which is a symmetrical circuit arranged to operated on alternate half-cycles and thereby provide the necessary controls to the gates of thyristors 15 and 16.

As will be seen, the resistor 38 is connected through resistors 50 and 51 to the bases of transistors 52 and 53. The collectors of these transistors are connected to a source of positive potential at terminal 54 through resistors 55 and 56. The collectors are also connected to the collectors of switching transistors 57 and 58; the emitters of both of these transistors being connected to ground. The emitters of transistors 52 and 53 are connected to the bases of transistors 59 and 60. The collectors of these transistors are connected to the positive supply terminal 54 through resistors 61 and 62 and load resistors 63 and 64. The output from the transistors is applied from the load resistors to the primary of pulse transformers 65 and 66. The emitters of transistors 59 and 60 are connected to ground. The bases of transistors 57 and 58 are driven from switching transformer 67. One terminal of this transformer is connected through diode 68 and resistor 69 to the base of transistor 57. The base of transistor 57 is also connected to ground through resistor 70 and Zener diode 71. In a similar manner, the opposite terminal of transformer 67 is connected through diode 72 and resistor 73 to the base of transistor 58, which base is connected to ground through Zener diode 74 and resistor 75. The centre point of the secondary transformer 67 is connected to the positive supply terminal 54 and the primary is provided with 60-cycle alternating current as indicated; this current being from the same source as other 60-cycle supplies in the system. The junction point of resistors 61 and 63 and resistors 62 and 64 is by-passed to ground through capacitors 77 and 76 respectively. The secondary of transformer 66 is connected between the gate and cathode of thyristor 16 and the secondary of transformer 65 is connected between the gate and cathode of thyristor 15.

The firing circuit just described is associated with thyristors 15 and 16. A similar circuit is of course required in association with thyristors 12 and 13, which circuit corresponds exactly with that shown and utilizes, insofar as possible, common waveforms and components. The basis of differentiation between the circuits is the bias supplied to the operational amplifier 28 and the corresponding amplifiers in further firing circuits.

OPERATION

Let us assume that a 60-cycle supply, as shown at A in FIG. 2, is provided for transformer 10 and the system is in operation providing current to the load consisting of lamps 18, 18a, etc. with the switches in the condition shown producing an effective load resistance as shown at time period t.sub.1 of curve B in FIG. 2. Waveform C in FIG. 2 is intended to be illustrative only. It will be seen that the solid line represents the ramp, while the dotted line represents the integrated output from amplifier 29. The polarity of this latter should be disregarded however, since it effectively illustrates the zero potential level of the combined signal indicating the point of transition from positive to negative value. At the point of intersection, the potential at the input of amplifier 31 changes polarity. It will therefore be seen that the polarity of the output from amplifier 31 varies from negative to positive at a time determined by the combination of the ramp and the integrated error signal.

Let us now consider the operation of the multivibrator consisting of amplifier 37 and its associated components. Let us assume that the output terminal of amplifier 37 is in its most positive state and has been in this state long enough to charge up capacitor 43 to a point where the inverting input terminal of the amplifier connected to capacitor 43 has just reached the potential of the non-inverting input terminal connected to resistor 49. The output will suddenly decrease and the output terminal of the amplifier will go to a negative value depending on its design. It remains in this condition until capacitor 43 discharges through diode 41 and resistor 39. When capacitor 43 has discharged to a sufficiently low level, the amplifier once again reverses its condition and the output becomes positive. This reversal of positive to negative condition occurs repetitively as long as the non-inverting input terminal of amplifier 37 is permitted to go positive, and occurs at a periodicity determined by resistors 39 and 40. It will be noted that one resistor, i.e. 39, determines the discharge rate and another resistor 40 determines the charge rate because of the inclusion of diodes 41 and 42.

If, however, the potential at the junction of resistors 35 and 36 is negative, the non-inverting input terminal to amplifier 37 is held at a negative potential. This negative potential at the non-inverting input to amplifier 37 produces a negative potential at the amplifier output, which charges capacitor 43 to a negative potential through resistor 39 and diode 41. The charge on capacitor 43 is limited by diode 44 and Zener diode 45 so that the magnitude of the potential at the non-inverting input is greater than that at the inverting input and the multivibrator action is arrested. In the absence of Zener diode 45, the capacitor would continue charging until it reaches the amplifier saturation voltage which would prevent the multivibrator resuming operation in a normal manner. As soon as the output from amplifier 31 goes positive, the multivibrator circuit is permitted to operate and produces a series of pulses. These pulses continue as long as the output from amplifier 31 is positive. The output pulses are applied as previously indicated through resistor 38 to the pulse forming network. The operation of the pulse forming network is as follows.

Assuming a series of pulses is applied through resistor 38 to the base of transistor 52 for example, this would normally cause transistor 52 to become conductive. However, it will be noted that transistor 57 is placed between the collector and ground on transistor 52 so that if transistor 57 is switched on, transistor 52 cannot be switched on. The operation of transistor 57 is determined by the switching circuit which includes transformer 67. It will be seen that the centre tap of the secondary of this transformer is supplied with a positive potential. In addition, let us assume that the secondary of the transformer is providing positive potential to diode 68 through resistor 69 to the base of transistor 57. This will cause transistor 57 to be conductive and transistor 52 therefore can not be operative. Zener diode 71 is provided to protect transistor 57 from the voltages produced by the combination of the supply voltage from terminal 54 and the output from the secondary of transformer 67. In order to provide rapid switching it is desirable that the output from transformer 67 be large thus producing an abrupt rise of voltage to a point where transistor 57 becomes conductive. On the other hand, it is not desired that large voltages be applied to the base of the transistor and these voltages are instead absorbed through the Zener diode 71.

When the polarity of the secondary of transformer 67 reverses, a negative potential appears on its secondary. This causes transistor 57 to be cut off and in this condition transistor 52 can become operative. With the pulses applied to its base through resistor 50, it then applies a similar series of pulses to the base of transistor 59. This transistor in turn conducts a series of current pulses which appears across its load resistor 63 and is applied to pulse transformer 65. These pulses of the form shown at C' in FIG. 3 in turn are applied to the gate and cathode of thyristor 15 causing the thyristor to commence conducting.

It will be seen therefore that the point of conduction of the thyristor is determined by the time of conduction of transistor 52, which can operate only during a particular half-cycle which is arranged to be the conductive half-cycle of thyristor 15, and also during the period which the multivibrator circuit produces pulses, this period being determined in time by the error signal and its point of coincidence with the ramp produced by the ramp generator. It will therefore appear that the point of conduction of thyristor 15 is determined by the current passing through current transformer 19. In a similar manner, the conducting period of thyristor 16 is also so determined. Thyristors 12 and 13 are similarly controlled by a similar circuit, the only difference being the adjustment of potentiometer corresponding to potentiometer 30, which determines the thyristor to be connected depending on the current required. Under circumstances where the load is not connected as shown, but instead is in its non-operative condition; that is, with contacts 20a and 20b open, and contacts 20d and 20c closed, a short circuit is applied to the secondary of transformer 14. This causes a current in the current transformer 19 whch rapidly rises to and exceeds the value established by potentiometer 26.

As shown by the dotted line in curve C of FIG. 2, the integrated error signal changes rapidly in a positive direction so that the effective point of intersection between the ramp and the integrated signal is delayed to a point late in the cycle. Therefore, the effective potential at the input of amplifier 31 only becomes negative late in the cycle and therefore the point of conduction of thyristors 12 and 13 is late in the cycle. It will be noted that the dash-dot line which represents the point of conduction of thyristors 15 and 16 goes right off the graph. These thyristors do not conduct under this condition, and only thyristors connected to the lower potential taps are fired. The various conductive conditions are illustrated in FIG. 2

As the load resistance changes as shown at B, current errors occur which when integrated by amplifier 28 and amplified by amplifiers such as 29, produce reference represented by the broken lines on C. The dotted line represents the reference used to control thyristors 12 and 13 while the dash-dot line represents the reference used to control thyristors 15 and 16.

In time period t.sub.1 the dotted line cuts the ramp early in the cycle and causes thyristors 12 and 13 to fire. Later in the cycle the dash-dot line also cuts the ramp and thyristors 15 and 16 are fired. The resultant voltage and current waveforms are shown at D and E respectively and the pulse trains used to fire thyristor 15 for example are shown at C'.

In time period t.sub.2 the load resistance increases and more voltage is required to produce the same current. Thyristors 12 and 13 fire for the whole cycle while thyristors 15 and 16 fire for a substantial portion of the cycle.

In time period t.sub.4 the load resistance is zero (with contacts 20d and 20c closed). The error signal is such that the amplifier associated with thyristors 15 and 16 is inoperative (due to its bias) and only thyristors 12 and 13 are fired. Since these are feeding a reactive load they conduct through the zero transistion. The series of pulses as shown at C' or a prolonged firing signal is used to ensure thyristors turn on in this period.

It should be evident that this system as illustrated is only one specific example of our invention. Only two thyristor pairs have been shown on transformer 10, but it is evident that a plurality of thyristor pairs could be included and connected to various transformer taps depending upon the degree of control required. The more thyristor pairs provided the less waveform distortion will be produced. It will also be seen that load transformer 14 is necessary in the present situation only because of the voltages and currents being controlled. Recognizing the voltage limitations of the thyristors, it was necessary that they be operated at a restricted voltage rating while the load was operated at a higher voltage. If this were not the case, transformer 14 could have been omitted. It will also be understood that the specific circuitry for producing the pulses, while preferred, is not the only possible arrangement.

It is also noted that the square law device designated 21 which produces a signal proportional to the square of the current in current transformer 19 may take various forms. For example, it may be produced by a proper arrangement of a Hall generator. Integrated devices are available however which will produce a square law signal in response to an input signal.

Finally, it is noted that the whole apparatus is assumed to be operated on a 60-cycle alternating supply, and while it is necessary that the various supplies all be from the same source, there is no reason why it should not operate at some other frequency dependent on the available source of supply.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An alternating current regulation system comprising a supply transformer supplied with alternating current having at least two secondary taps, means to selectively connect a load to one of said taps during portions of the cycle and thereby regulate the current supplied to said load, said means including a pair of oppositely-poled thyristors connected to each tap, means to produce a first signal representative of the square of the currents supplied to the load, means to produce a second signal representative of the desired load current, means to compare said first and second signals to produce an error signal, means to integrate said error signal, means to compare said integrated error signal with a first reference signal and produce a first output signal when said error signal exceeds said first reference signal, a source of cyclic sawtooth waveforms in phase synchronism with the alternating current being regulated, means to combine said output signal with said sawtooth waveform and produce a first control signal when said sawtooth waveform is equal in absolute value to said output signal, a first oscillator rendered operative by said first control signal, means to apply the output of said first oscillator to the firing electrodes of a first of said pairs of oppositely-poled thyristors in the proper phase to cause the proper one of said first pair of thyristors to conduct and connect a first tap to said load, means to compare said integrated error signal with a second reference signal and produce a second output signal when said error signal exceeds said second reference signal, means to produce a second control signal, a second oscillator rendered operative by said second control signal and means to apply the output of said second oscillator to the firing electrodes of a second of said pairs of oppositely-poled thyristors in the proper phase to cause the proper one of said second pair of thyristors to conduct and connect a second tap to said load.

2. A regulation system as claimed in claim 1 wherein said second reference signal is greater than said first reference signal and the voltage supplied by said second tap is greater than the voltage applied by said first tap.