GB2098414A

GB2098414A - Parallel operated inverters

Info

Publication number: GB2098414A
Application number: GB8213172A
Authority: GB
Original assignee: BRAMMER DYNAMICS Ltd
Current assignee: BRAMMER DYNAMICS Ltd
Priority date: 1981-05-07
Filing date: 1982-05-06
Publication date: 1982-11-17

Abstract

Inverters 1 and 2 are connected in parallel at their a.c. outputs and each inverter has at or before its d.c. input side at least one circuit element 5, 6, 7 which assists in achieving current matching in controllable rectifier elements of the corresponding inverters which carry current simultaneously when the inverters are operating. In one embodiment two PWM motor drive inverter units 1 and 2 are connected in parallel between a three-phase constant frequency input 3 and a three-phase variable frequency output 4. Each unit comprises a diode rectifying bridge 5, a d.c. intermediate circuit comprising an inductance 6 and a capacitor 7, and a PWM thyristor bridge 8. The bridges 8 are controlled synchronously and good current matching in the simultaneously conducting thyristors of the two bridges 8 is achieved with the aid of the internal resistance of the diodes of the bridges 5, the inductances 6 and the capacitors 7, and output chokes 9 connected in the output connections of the two bridges 8. The corresponding output chokes may be coupled as in Fig. 3. <IMAGE>

Description

SPECIFICATION Improvements in or relating to inverters This invention relates to improvements in or relating to inverters and is particularly, but not exclusively, applicable to PWM inverters.

An inverter usually comprises a bridge circuit of controllable rectifier elements, e.g. thyristors. In use the inverter receives d.c. at its input side and outputs a.c. produced by suitable control of the controllable rectifier elements. A preferred mode of control of these elements is PWM (pulse width modulation).

If it is desired to increase the output current and power capability of an inverter, conventionally two different methods are employed. Firstly, the controllable rectifier elements may be replaced by similar elements with a greater current carrying capacity. Large elements are expensive however, and have a longer switching time than do smaller elements. Secondly, each of the elements may be supplemented by connecting in parallel with it a further such element. This measure is cumbersome and costly however, because there must also be added series resistances and other circuit elements in each pair of paralleled elements in order to achieve matched current flow in the elements of a pair. Moreover, these resistances absorb power which could otherwise be fed from the outputs of the inverter.

According to the present invention there is provided an inverter arrangement in which a plurality of corresponding inverter units are connected in parallel at the a.c. output side, each inverter unit comprising at or before its d.c. input side at least one circuit element which assists in achieving current matching in controllable rectifier elements, of the different corresponding inverter units, which carry current simultaneously when the inverter arrangement is in operation.

A preferred embodiment of the present invention comprises two identical frequency changer units connected in parallel, thereby to provide twice the current and power output capability of a single such unit. Each unit comprises a rectifier bridge followed by a d.c.

intermediate circuit including smoothing elements, followed by the inverter unit. In this particular case the internal resistance of the rectifier bridge elements and the electrical properties of the smoothing elements may assist in achieving current matching as aforesaid.

For a better understanding of the present invention and to show how it may be put into effect reference will now be made, by way of example, to the accompanying drawing in which the three figures show three embodiments of the present invention.

Figure 1 shows two identical standard frequency changers 1 and 2 connected in parallel between a common constant frequency 3-phase input 3, and a common variable frequency 3phase output 4.

Each frequency changer comprises a three phase rectifier bridge 5 comprising uncontrolled diodes, a d.c. intermediate circuit comprising a smoothing inductor 6 and capacitor 7, and a three phase preferably PWM inverter 8 comprising controlled rectifier elements such as thyristors.

The outputs of the two inverters 8 are connected in parallel by interconnection of the mid-points of the respective bridge arms. The three 3-phase output connections thus provided can be used to supply, for example, a speed-controlled 3-phase induction motor.

The arrangement is put in operation with the two inverters being controlled in synchronism with one another.

The internal resistance of the diodes of the bridges 5 and the action of the inductors 6 and capacitors 7 provide good current matching in the controllable rectifier elements, of the different inverters 8, which carry current simultaneously when the inverter arrangement is in use. Thus, good current matching is achieved between say the top left hand controllable rectifier elements on the one hand, and between say the bottom right hand controllable rectifier elements on the other hand when these four elements are conductive simultaneously in the pulse width modulation program. No costly, complex and current consuming additional resistances are required.

The standardly-provided components provide current matching.

The illustrated and described arrangement is simple to effect and avoids the aforementioned disadvantages of the prior art. Although only two paralleled units are described and illustrated, the principle can be extended to more paralleled units.

Another way of providing current matching would be to provide suitable chokes in the 3phase inputs of the respective rectifier bridges 5.

In the parallel inverters arrangement described and illustrated in respect of Figure 1, the internal resistance of the diodes of the bridges 5 and the action of the inductors 6 and capacitors 7 provide a good current matching as described hereinbefore, but only in respect of slow current changes in the output waveform of the arrangement. That is to say, good current matching is achieved only in respect of current changes in the fundamental output current waveform, which has a frequency of say 48 Hz.

Because of the action of the capacitors 7, good current matching in respect of rapid current changes in the inverters 8 is not achieved. Such rapid current changes occur in the PWM switching of the thyristors in the inverters 8, which switching provides a c'arrier waveform, for the fundamental waveform, of say 600 Hz.

In order to improve the current matching in respect of the fast current changes required in the PWM technique, the circuit arrangement shown in Figure 1 can be modified as shown in Figure 2, by the addition of a series output choke 9 provided at the output of each phase of the two inverters 8. All chokes 9 are equal.

Figure 3 shows an alternative method of providing this type of inductance at the output of a two-inverter arrangement according to the invention, in which each corresponding pair of output phases of the two inverters 8 are combined in output chokes 10 wound with common coupling. Of course the Figure 3 type of arrangement could not be employed to combine the outputs of three inverters, a Figure 2 type of arrangement would be employed in that case.

It is to be noted that alhtough it is advantageous to provide output inductance as typified by Figures 2 and 3, this is by no means an essential measure, even with PWM inverters.

However, the employment of such output inductance will improve the efficiency and output power of a paralleled-inverters unit.

Although in each of Figures 1 to 3 only two inverters are shown in parallel it is quite possible to extend the principle to more inverters. For example, three 75 H.P. motor drives, each comprising a rectifier and a PWM inverter may be provided and coupled as shown in Fig. 2, to provide a single nominally 225 H.P. drive unit.

It is possible, employing a single design and rating of motor drive unit, to provide a multiple paralleled unit with two, three or even more times the output capacity of the single unit. In practice this means a reduced design time based on customer specification; circuit elements are standardised which is good from the point of view of bulk buying inter alia; and the employment of standard frequency changer modules makes for ease and economy of serviceability.

Claims

1. An inverter arrangement in which a plurality of corresponding inverter units are connected in parallel at the a.c. output side, each inverter unit comprising at or before its d.c. input side at least one circuit element which assists in achieving current matching in controllable rectifier elements, of the different corresponding inverter units, which carry current simultaneously when the inverter arrangement is in operation.

2. An inverter arrangement according to claim 1, wherein the at least one circuit element comprises an inductance in a d.c. circuit preceding the inverter.

3. An inverter arrangement according to claim 1 or 2, wherein the at least one circuit element comprise diodes of a rectifier bridge preceding the inverter.

4. An inverter arrangement according to any preceding claim, wherein the at least one circuit element comprise chokes at the input side of a rectifier bridge preceding the inverter.

5. An inverter arrangement according to any preceding claim, wherein each inverter unit comprises at its a.c. output side inductance which assists in achieving current matching in the controllable rectifier elements, of the different corresponding inverter units, which carry current simultaneously when the inverter arrangement is in use.

6. An inverter arrangement according to claim 5, wherein each inverter unit comprises a choke provided in each phase output of the unit.

7. An inverter arrangement according to claim 5, comprising two inverter units connected in parallel at the output side, each pair of corresponding output phases thereof providing a single phase output via output chokes wound with common coupiing.

8. An inverter arrangement substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

9. An inverter arrangement substantially as hereinbefore described with reference to Figure 1 as modified in accordance with Figure 2 or 3 of the accompanying drawings.

10. An inverter arrangement according to any preceding claim, wherein the inverters are parts of respective frequency changing motor drive units.

1 An inverter arrangement according to any preceding claim, wherein the inverters are PWM inverters.