Low Inductance GTO Switching Circuit for an Inverter

1. Field of the Invention

The present invention relates to a converter arrangement which includes a directed voltage circuit and at least one inverter circuit.

2. Discussion of Background

Prior art is known from the publication by W. McMurray, Circuit d'aide a la commutation des GTO (circuit using GTO switching), in the French journal: electronique de PUISSANCE 11, September 1985, pages 12 and 13. In that publication, an inverter circuit with 2 GTO thyristors is specified which are connected in series via a reactor. The load is connected to the center tap of the reactor. The snubber capacitors and snubber diodes of the two GTO thyristors are connected to one another via a discharge resistor. In order to minimize parasitic inductances, a capacitor is provided on the direct voltage side of the inverter.

The invention achieves the object of increasing the direct voltage permissible at the inverter without increasing the number of the inverter valves.

An advantage of the invention consists in that the power of the inverter can be increased along with the increase in the inverter direct voltage. This results in lower costs per power unit.

The increase in the permissible direct voltage is achieved by design measures for reducing the parasitic inductances. These measures are based on the common idea of keeping electrical loops as small as possible.

In accordance with an advantageous embodiment of the invention, when GTO thyristors with a turn-off current of 2 kA are used a combination of the measures according to the invention permits an increase in the applied direct voltage of normally 3.2 kV to 3.5 kV. This is achieved by a reduction in parasitic inductances of normally 0.6 .mu.H to 0.15 .mu.H.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 shows an application-oriented converter circuit with a dc link and an inverter in a 3-phase design,

FIG. 2 shows a detailed circuit of the inverter in accordance with FIG. 1 for one phase of a load-side alternating current,

FIG. 3 shows a block diagram of the arrangement of the components in the inverter circuit in accordance with FIG. 2, with closed link capacitor bank,

FIG. 4 shows a discharge resistor of the inverter circuits in accordance with FIGS. 2 and 3,

FIG. 5 shows a representation of sections of the arrangement of components of the inverter circuit in accordance with FIG. 3,

FIG. 6 shows a block diagram of parallel-connected capacitors of the link capacitor bank in accordance with FIGS. 1 and 3,

FIG. 7 shows a circuit arrangement of capacitors, connected in parallel and in series, of the capacitor bank in accordance with FIGS. 1 and 3, and

FIG. 8 shows a block diagram of the time dependence of anode-cathode voltage and anode-cathode current of a GTO thyristor of the inverter in accordance with FIGS. 1-3.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 shows as an example of the application of the converter arrangement according to the invention a 3-phase asynchronous machine (5) which is fed per ac phase by a converter (1-4) via a load current conductor (L1-L3) in each case. The converter (1-4) consists of a rectifier (1), an inverter (4) and a dc link with a link reactor (2) and a link capacitor bank (3).

FIG. 2 shows the circuit of the inverter (4) for one ac phase. The circuit for the two other ac phases is identically constructed. (T1) and (T2) designate turn-off valves or GTO thyristors with anti-parallel diodes, which are connected in series via two reactance coils (10, 11) to an inductance of 5 .mu.H in each case. The GTO thyristors (T1, T2), are designed for a turn-off current of 2 kA and a direct voltage of 4.5 kV. A load current conductor (L1) is connected to the electrical tie point of the 2 reactance coils. Instead of 2 reactance coils (10, 11), it is also possible to use one reactance coil with center tap (not represented). These reactance coils (10, 11) can also be eliminated in a series circuit of 2 inverters via a shared load.

A snubber capacitor (C1) or (C2) is connected in series with a snubber diode (D1) or (D2) in a parallel branch to each GTO thyristor (T1, T2). The capacitance of the snubber capacitors (C1, C2) is 2.5 .mu.F in each case. The two snubber capacitors (C1, C2) and snubber diodes (D1, D2) are connected to one another via a discharge resistor (12). (6, 9) or (7, 8) and (12d) designate undesired, parasitic leakaqe inductances of the supply leads or of the snubber capacitors (C1, C2) and of the discharge resistor (12). Without the measures according to the invention, the leakage inductances (6-9) amount to approximately 0.1 .mu.H in each case, while the leakage inductance (12d) of the discharge resistor (12) is approximately 0.5 .mu.H.

FIG. 3 shows the arrangement of the components in accordance with FIG. 2 in connection with a link capacitor bank (3), but without the leakage inductances (6-9, 12d). (13-16) designate metallic heat sinks which are arranged on both sides of the GTO thyristors (T1, T2) and dissipate the heat losses thereof. The heat sinks (13-16) can be operated using air, oil or water cooling. The GTO thyristors (T1, T2) and the heat sinks (13-16) are situated together with an insulator (32) in a connecting strainer (33) indicated by dashes, which is acted upon from both sides by equally strong but oppositely directed elastic forces (K1, K2), in order to ensure good thermal conduction between the GTO thyristors (T1, T2) and their heat sinks (13-16).

(23) and (24) are respectively positive and negative current conductors for connection of the capacitor bank (3), which extend essentially parallel to one another and are respectively connected to the anode-side heat sink (14) of the GTO thyristor (T1), which is at a positive potential, and to the cathode-side heat sink (15) of the GTO thyristor (T2), which is at a negative potential. The snubber diodes (D1, D2) are respectively mounted to conduct heat effectively on the cathode-side heat sink (13) of the GTO thyristor (T1) and on the anode-side heat sink (16) of the GTO thyristor (T2), as can be seen better from FIG. 5. The snubber capacitors (C1, C2) are arranged at a slight lateral spacing from the heat sinks (14) and (15), respectively; they each have a capacitor cover (17), which is made of copper or is a good electrical conductor and surrounds the snubber capacitors at least partially. In order to keep leakage inductances low, the discharge resistor (12) is arranged at a slight distance symmetrically with respect to the two capacitor covers (17), and connected to the latter.

FIG. 4 shows the discharge resistor (12) diagrammatically in cross-section. To enable good ventilation, a resistor body designated by (12a) is constructed by means of spaced resistor plates. A copper cover or sheet-metal shielding cover (12b), which contacts the resistor body electrically on the end faces, preferably over the entire end face, is slipped over the resistor body (12a) from above and below, respectively. Connected electrically to the sheet-metal shielding covers (12b) are connecting plates (12c) which lead in the gap between the resistor plates to the end faces of the resistor body (12a), so that the current is led to these end faces both from outside and from inside, cf. the arrows drawn in. The current flows through the resistor body (12a) from the upper end face to its lower end face, and is then removed via the lower sheet-metal shielding cover (12b) and the connecting plate (12c) connected thereto.

FIG. 5 shows in a section the design of the construction of the circuit arrangement in accordance with FIG. 3. (22) designates a disk-shaped insulator between the two snubber capacitors (C1, C2), and (25) designates a disk-shaped insulator between link capacitors (C11-Cn1, Cn1'-C11') connected in series. The thickness of the insulator (25) is .ltoreq.4 mm/kV, preferably .gtoreq.1.5 mm/kV, in the example 3 mm. The link capacitors (C11-Cn1, Cn1'-C11') are connected to the positive or negative current conductor (23, 24), and connected to one another by connecting bars (26-28) extending parallel to one another. The end bar (28) is configured in the shape of a U. The link capacitors (C11-Cn1) or (Cn1'-C11') are arranged in opposite planes in such a way that the connection elements of the capacitors are opposite one another. The central axes of the oppositely arranged capacitors (C11-C11'; C21, C21'; . . .) need not necessarily be identical; they can also be arranged slightly offset with respect to one another (not represented).

The snubber capacitor (C1) is surrounded in its upper part by a closely fitting capacitor cover (17) and in its lower part by a capacitor insulating cap (18), which has an opening in the center for an electrical terminal. This terminal is electrically connected to the heat sink (14) via a U-shaped connecting bracket (20) whose U-bow is situated above the GTO thyristor (T1). The capacitor cover (17) is connected via a conductive U-shaped connecting bracket (19) to the diode (D1), and the latter is connected to the heat sink (13) via a valve shielding bracket (21). The valve shielding bracket (21) is curved above the GTO thyristor (T1) and spaced from the connecting bracket (20) by the required insulating spacing. As a result, the space adjacent to the GTO thyristor (T1) is kept largely free of electric fields.

FIG. 6 shows a parallel capacitor circuit of link capacitors (C11-C1m) and (C11'-C1m') of the link capacitor bank (3), link capacitors which are connected in parallel being arranged with respect to the representation in FIG. 5 in a plane perpendicular to the plane of the drawing. (29) and (30) designate capacitor connecting bars extending parallel to one another, and (31) designates an insulator situated therebetween, corresponding to the insulator (25).

FIG. 7 shows a low-inductance arrangement of link capacitors (C11-C16, C21-C26, Cn1-Cn6) connected in parallel and in series of the link capacitor bank (3) in accordance with FIG. 1, which arrangement is of particular interest when the space is restricted in width. The link capacitors (C11-C16, Cn1-Cn6) to be connected in parallel are arranged in each case in 2 rows offset with respect to one another, a (2n)th capacitor (C12, C14, C16; Cn2, Cn4, Cn6) being placed in a 2nd row partially in the gap between the (2n-1)th and the (2n+1)th capacitor (C11, C13, C15; . . . Cn1, Cn3, Cn5) in a row 1st n=1, 2, . . . being integers.

The link capacitors (C11-C16, . . . Cn1-Cn6) to be connected in parallel are electrically connected by finger sheets (Fl-Fn) with sheet metal fingers (F12, F14, F16; . . . Fn1, Fn3, Fn5), which are offset with respect to one another and reach into the gaps of the respective other capacitor row. Finger sheets (F2-Fn-1) for a series circuit have sheet metal fingers (F21, F23, F25) or (F22, F24, F26) offset with respect to one another on opposite sides. The further capacitor parallel circuits (C21-C26; . . . Cn1-Cn6) to be connected in series with a lst capacitor parallel circuit (C11-C16) are identically constructed and arranged as the 1st. The last finger sheet (Fn) is bent in the shape of a U, cf. the capacitor connecting bar (28) in FIG. 5.

Instead of being rectangular, the lugs of the sheet metal fingers (F12-Fn5) on the respective finger sheet (Fl-Fn) can also be rounded. It is important that adequate insulating spacings are ensured.

Plotted in FIG. 8 on the ordinate is the voltage (U) and the current (I) and on the abscissa the time (t) in arbitrary units. (U.sub.AK) designates the temporal variation of the anode-cathode voltage of a GTO thyristor (T1, T2), and (I.sub.AK) designates the temporal variation in the anode-cathode current of the same thyristor. After the arrival of a turn-off signal at the gate electrode of the turn-off thyristor at an instant (t1), the thyristor's gate electrode-cathode current has reached 1/10 of its minimum value (not represented), while (U.sub.AK) and (I.sub.AK) are still unchanged. The anode-cathode current (I.sub.AK) reaches a first minimum at an instant (t2). At the same time, the anode-cathode voltage (U.sub.AK) reaches a first maximum, which is not permitted to exceed a first limit value (U.sub.G1) set by the design. At an instant (t3), (U.sub.AK) reaches a 2nd maximum, which is not permitted to exceed a 2nd limit value (U.sub.G2), likewise set by the design.

The achievement of the measures for reducing the parasitic inductances is that the 2nd maximum, in particular, of (U.sub.AK) is reduced, cf. the dotted curve section denoted by (U.sub.K '). It is possible for the link direct voltage now to be increased by this voltage difference, and for the power of the inverter to be increased in proportion to this difference.

A particularly low-inductance connection of the link capacitor bank (3) is ensured by the arrangement of the plus/minus terminals (23, 24) in the middle of the heat sink connecting strainer (33) instead of the usual end arrangement. The leakage inductance of the snubber capacitors (C1, C2) is reduced principally by means of the capacitor covers (17), and those of the discharge resistor (12) by the two sheet metal shielding covers (12b). The reduction of all 5 parasitic inductances (6-9, 12d) reduces the voltage stress of the GTO thyristors (T1, T2), to the same extent, and permits an increase in the link voltage. However even individual measures, such as with the discharge resistor, for example, can already permit a substantial increase in direct voltage.

In the practical embodiment, a total reduction of approximately 0.45 .mu.H is achieved in the leakage inductances (6-9, 12d), which permits the link voltage to be raised by approximately 300 V from 3.2 kV to 3.5 kV.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.