EP2319059B1 - Tap switch with semiconductor switching elements - Google Patents

Abstract

An on-load tap changer with semiconductor IGBT switching elements for uninterrupted switching over between winding taps of a tapped transformer, has two load branches connectable with the respective winding taps and each load branch is electrically connected with a common load output line through a respective series circuit consisting of two oppositely connected IGBTs. A diode is connected parallel to each IGBT, and the two diodes in each load branch are connected oppositely to one another. A respective mechanical switch is connected in series with the series circuit of IGBTs and parallel diodes in each load branch. A respective varistor is connected parallel to each parallel circuit of IGBT and diode, and the varistors are so dimensioned that the respective varistor voltages are lower than the maximum blocking voltage of the respective parallel IGBTs but higher than the maximum instantaneous value of the tap voltage.

Description

The invention relates to a tap changer with semiconductor switching elements for uninterrupted switching between winding taps of a tapped transformer.

A tap changer with semiconductor switching elements, which is designed as a hybrid IGBT switch is out of the WO 01/22447 known. The tap changer described there operates on the principle of a Lauflastschalters, which can be dispensed with a power storage. It has as a hybrid switch a mechanical part and an electrical part.

The mechanical part, which is the real object of WO 01/22447 is, has mechanical switching contacts; the central part is a movable sliding contact, which is moved along a contact rail connected to the star point by means of a motor drive, thereby connecting fixed contact elements. The actual load switching itself is done by two IGBT's with four diodes each in Grätzschaltung.

This known concept of a hybrid switch is mechanically complex and demanding to ensure the necessary load switching exactly at the zero crossing of the load current.

From the WO 97/05536 a further IGBT switching device is known in which the taps of the control winding of a power transformer via a series circuit of two IGBT's are connected to a common load dissipation.

This known switching device operates on the principle of pulse width modulation; There is a limitation of the circulating current through the transient reactive reactance (TER) of the step winding.

This known circuit arrangement and the underlying switching principle require a specific adaptation of the tap changer to the respective tap transformer, which is to be connected. In other words: tapped transformer and tap changer are coordinated and act together electrically. This known switching device is thus not as a separate, universally applicable device to produce. WO 97/05536 discloses a tap changer according to Oberbergriff of claim 1.

From the GB-A-2424766 Finally, various circuit arrangements for a tap changer are known, containing variously connected varistors. In one embodiment, varistors are connected in parallel to the respective switching elements and serve for voltage distribution.

The object of the invention is to provide a tap changer of the type mentioned, which is simple in construction, has a high reliability and in which it is not necessary to have to switch only exactly at the zero crossing of the load current. Furthermore, it is an object of the invention to provide such a tap changer, which does not need to be specially adapted to the respective rated load current and the respective windings of the connected stage transformer, but quasi "off the shelf" can be connected as a functional device to a variety of tapped transformers.

This object is achieved by a tap changer having the features of the first claim. The subclaims relate to particularly advantageous developments of the invention.

The invention is based on two switching units, each switching unit each consisting of two associated, antiparallel IGBT's. Each individual IGBT is assigned a parallel connected varistor. The varistor is dimensioned such that the varistor voltage is less than the maximum blocking voltage of the respective parallel IGBT, but greater than the maximum instantaneous value of the step voltage.

Particularly advantageous are the two associated IGBT's an antiparallel switching unit clamped together in the form of a compact stack.

Furthermore, it is particularly advantageous to position the respective varistor in the sense of a low-inductance as possible parallel path immediately adjacent to each IGBT and to integrate into the stack. In this way, extremely short line connections between the IGBT and the parallelliegenden varistor can be realized. This arrangement allows a very fast, "hard" shutdown of the current flowing through the IGBT load current with commutation within 0.1 ... 1 microseconds to the extremely inductively connected varistor, even with a full instantaneous value of the load current itself only a very small response delay in has ns range.

The "hard-switching" of the IGBT reduces the turn-off power dissipated in the IGBT decisively and enables the switching concept of a tap changer (OLTC) switching at any arbitrary value of the instantaneous load current, without additional overshoot impedance in the OLTC, as described in detail below without the need to know the step-around leakage reactance, without the need to adapt the OLTC to the particular rated load current or step voltage, and without the need for μs accurate timing of the turn-off and take-over IGBT switching group.

Although are from the DE 101 18 743 A1 and numerous other publications already known varistors in conjunction with IGBT's. In the prior art, however, they serve exclusively to protect semiconductors from overvoltages, ie they merely have a voltage-limiting function.

In the invention, however, the function of the parallel to each IGBT varistor is another: After commutation of the impressed, driven by the mains load current from the disconnecting IGBT to the varallelliegenden varistor (small commutation), builds from the load current flowing varistor according to its IU characteristic a voltage which shows a relatively small dependence on the instantaneous value of the current and remains practically constant during the switching process of the OLTC.

The varistors are particularly advantageously dimensioned such that the varistor voltage, which results when loaded with the peak value of the maximum current, still has a sufficient safety margin to the maximum reverse voltage of the IGBTs.

On the other hand, the clamping voltage of the varistors (U _Var at 1 mA) must be well above the peak value of the maximum step voltage so that the load current can commute from the disconnecting OLTC side over the step voltage to the side taking over the load current (large commutation circuit).

The difference .DELTA.U between instantaneous value of the voltage drop across the varistor and the instantaneous value of the step voltage causes the commutation of the load current via the stray inductance of the tap winding and the line inductances on the receiving side of the tap changer and determines the di / dt of the commutation process by the special dimensioning of the varistors. dt = ΔU / L _Kom ).

• Übernahme des Laststromes von den hart abschaltenden IGBTs
• Erzeugung eines Spannungsabfalls, der unabhängig vom Momentanwert des Laststromes in einem Spannungsband zwischen der maximalen Sperrspannung der IGBTs und dem Scheitelwert der maximalen Stufenspannung liegen muss
• Bereitstellung einer Spannungs-Zeitfläche, die den Laststrom von der stromführenden Seite des Stufenschalters über die entgegen gerichtete Stufenspannung auf die übernehmende Stufenschalterseite kommutiert.

This clarifies that the varistors are not used in the context of the present invention, as known in the art, for reducing transient overvoltages. In the present invention, the varistors adopt the following functions, which are atypical of their type and not suggested by the prior art:

• Takeover of the load current from the hard-breaking IGBTs
• Generation of a voltage drop, which must be independent of the instantaneous value of the load current in a voltage band between the maximum blocking voltage of the IGBTs and the peak value of the maximum step voltage
• Providing a voltage-time area that commutates the load current from the live side of the tap changer via the opposing step voltage to the receiving tap changer side.

The invention results in a very simple and cost-effective dimensioning of power electronic switching groups, because the energy-absorbing volume in the case of the varistor is flexibly variable and unequal greater than the much smaller, more expensive and hardly variable in volume volume of the IGBT chip.

As a further positive effect of the load current control by the varistors, the provision of the required commutation voltage-time area by the varistors and the recording of the resulting loss energy also by the varistors results in a very large tolerance field with regard to the synchronization of the turn-off of the disabling IGBT group and the switch-on time of the acquiring IGBT group.

If, in the course of the operating years due to component aging and working point displacement in the control electronics, an overlapping or gaping switching behavior should be on the order of about ± 10 μs, this does not result in a functional hazard in the switching concept according to the invention.

• Option der Umschaltung bei jedem beliebigen Momentwert des Laststromes ohne thermische Überbeanspruchung der IGBTs
• Außerordentlich schneller Kommutierungsvorgang des Laststromes von Stufenschalterseite A → B oder B → A innerhalb von ca. 10 µs.
• Vermeidung störender Oszillationen
• Eine auftragsspezifische Anpassung jedes Stufenschalters an die konkreten Nenn-Stufendaten des Bestellfalles (Stufenspannung, Nenn-Durchgangsstrom, Streuinduktivität) entfällt, solange die Grenzwerte von Stufenspannung und Nenn-Durchgangsstrom nicht überschritten werden.
• Robustes, eigensicheres Kommutierungskonzept mit einem sehr großen Toleranzbereich in Bezug auf Schaltzeitdrift zwischen den beiden IGBT-Schaltgruppen. Keine Nachjustierung nach längerer Betriebszeit erforderlich.

In summary, the invention has the following advantages:

• Option of switching at any torque value of the load current without thermal overstressing of the IGBTs
• Extremely fast commutation of the load current from tap-changer side A → B or B → A within approx. 10 μs.
• Avoid disturbing oscillations
• An order-specific adaptation of each tap changer to the specific nominal step data of the order case (step voltage, nominal through-current, leakage inductance) is eliminated as long as the limit values of step voltage and rated through-current are not exceeded.
• Robust, intrinsically safe commutation concept with a very large tolerance range with regard to switching time drift between the two IGBT switching groups. No readjustment required after a longer period of operation.

Figur 1

zeigt die Schaltung eines ersten erfindungsgemäßen Stufenschalters.

Figur 2

zeigt die Schaltung eines zweiten, im Rahmen der Erfindung abgewandelten Stufenschalters.

The invention will be explained in more detail by way of example with reference to figures.

FIG. 1

shows the circuit of a first tap changer according to the invention.

FIG. 2

shows the circuit of a second, modified within the scope of the invention tap changer.

As in FIG. 1 shown, each of the two winding taps tap n and tap n + 1 via a mechanical switch DS _a and DS _b with a series circuit of two oppositely connected IGBTs I _an and I _ap on the side n and I _bn and I _bp on the Page n + 1 connected to the tap changer lead. Parallel to each of the two series-connected IGBT's I _and I _ap one and I _bn and I _bp the other hand is in each case a diode d _of d _ap or d _bn, d _bp connected. In this case, the diodes of one side, ie d _an and d _ap and d _bn and d _{bp are} opposite to each other, ie connected in the opposite forward direction.

Again, parallel to each of these parallel circuits of the IGBT and the diode, a varistor V _an , V _ap and V _bn , V _{bp are} respectively provided.

Finally, even the, the entire switching device in stationary operation each bridging, permanent main contacts each side MC _a and MC _b are shown. The IGBTs of both sides I _an , I _ap ; I _bn , I _bp are driven by a common, only schematically illustrated, known from the prior art IGBT driver.

In the following, a switching sequence of, for example, tap n to tap n + 1 is explained in more detail: In the basic position, the load current flows via the permanent main contact MC _a from tap n to the tap changer lead Y.

As a first step of the switching sequence, the enable contacts DS _a and DS _{b are} closed.

Then starting voltage is applied to the gates of the IGBTs I _and I _ap. Now opens the permanent main contact MC _a and commutates the load current I _L to the IGBT group I _an / I _ap.

After less than 10 ms current flow duration of I _L via the IGBT group I _to / I _ap , these IGBTs receive a switch-off command and the IGBT group I _bn / I _bp simultaneously (at least in the standard case) a switch-on command.

The voltage that builds up on the disconnecting IGBT is transferred to the parallel varistor. If ns, the clamping voltage of the varistor is achieved after a few 100, the varistor begins to conduct, which uses the takeover of the load current of the IGBT's I _and I _ap.

The varistor according to the invention is dimensioned so that the voltage of the load current flowing through the varistor on the one hand below the max. Blocking voltage of the parallel IGBT, on the other hand moved above the maximum instantaneous value of the step voltage.

The excess of the instantaneous value of the varistor voltage over the instantaneous value of the step voltage causes the commutation of the load current with approximately constant di / dt from side A and a shift over the step voltage and the leakage inductance of the step winding L _σ (large commutation) with the same di / dt (in this Case positive) on the side B. Despite the continuously decreasing current flowing through the varistor on side A, the varistor voltage remains constant in a first approximation.

• Die Varistorspannung bricht zusammen, das transiente $${\mathit{L}}_{\mathit{\sigma }}\frac{\mathit{di}}{\mathit{dt}}$$
  
  verschwindet und an der IGBT-/Varistor-Gruppe A erscheint die Stufenspannung, die abhängig von der Polarität an einem blockierenden IGBT, der dazu parallelliegenden Diode und dem jeweils wiederum parallelliegenden Varistor ansteht. Selbst bei Belastung mit dem Scheitelwert der Stufenspannung lässt der Varistor noch keinen signifikanten Stromfluss zu.

After about 10 μs, the entire load current from the current-carrying side A varistor is commutated over to the side B conductive IGBTs. As the current of side A approaches 0, the voltage on switch group A changes fundamentally:

• The varistor voltage collapses, the transient $${\mathit{L}}_{\mathit{\sigma }}\frac{\mathit{di}}{\mathit{dt}}$$
  
  disappears and on the IGBT / varistor group A, the step voltage appears, which depends on the polarity of a blocking IGBT, parallel to the diode and the next parallel varistor. Even when loaded with the peak value of the step voltage, the varistor still does not allow significant current flow.

Less than 10 ms after the power electronic commutation of the load current from side A to side B, the permanent main contact MC _b closes and shunts the IGBT group B. Subsequently, the IGBTs I _bn / I _bp are switched to the non-conducting state via the gate drive.

The switching sequence ends with the opening of the mechanical release contacts DS _a and DS _b , which protect the IGBTs from the transient voltage stresses that may be effective at the tap winding.

In FIG. 2 a modified circuit of a tap changer according to the invention is shown, in which the two varistors each side V _on , V _ap and V _bn , V _{bp are combined} to form a respective common varistor V _a and V _b . In this case, the respective mechanical switch of each side DS _a or DS _b and the respective varistor of the associated side V _a and V _b also forms a series connection to the common load dissipation.

Claims (4)

1. On-load tap changer with semiconductor switching elements for uninterrupted switching over between winding taps of a tapped transformer, wherein the on-load tap changer has two load branches connectible with the respective winding taps, wherein the semiconductor switching elements are IGBTs (I_an, I_ap; I_bn, I_bp), wherein each of the two load branches is electrically connected with a common load output line by way of an arranged series circuit consisting of two oppositely connected IGBTs (I_an, I_ap; I_bn, I_bp) and wherein a diode (d_an, d_ap; d_bn, d_bp) is connected in parallel with each IGBT (I_an, I_ap; I_bn, I_bp), the two diodes in each load branch (d_an, d_ap or d_bn, d_bp) being connected oppositely to one another, characterised in that a respective mechanical switch (DS_a, DS_b) is connected in series with the series circuit of IGBTs (I_an, I_ap; I_bn, I_bp) and parallel diodes (d_an, d_ap; d_bn, d_bp) in each load branch, that a respective varistor (V_an, V_ap; V_bn, V_bp) is connected in parallel with each parallel circuit of IGBT (I_an, I_ap; I_bn, I_bp) and diode (d_an, d_ap; d_bn, d_bp) and that the varistors (V_an, V_ap or V_bn, V_bp) are so dimensioned that the varistor voltage thereof is lower than the maximum blocking voltage of the respective parallel IGBTs, but higher than the maximum instantaneous value of the tap voltage.
2. On-load tap changer according to claim 1, characterised in that each IGBT (I_an, I_ap; I_bn, I_bp) is constructionally combined to form a stack together with the varistor (V_an, V_ap; V_bn, V_bp) connected in parallel therewith and the diode (d_an, d_ap; d_bn, d_bp).
3. On-load tap changer according to claim 1, characterised in that the two varistors (V_an, V_ap or V_bn, V_bp) provided in the same load branch are combined to form a single varistor (V_a, V_b).
4. On-load tap changer according to any one of claims 1 to 3, characterised in that a respective mechanical main latching contact (MC_a, MC_b) is provided in parallel with each of the two load branches.

Images