HK1151130B - Method for switching without any interruption between winding taps on a tap-changing transformer - Google Patents

Description

The invention relates to a method for continuous switching using semiconductor switching elements between winding sockets of a step transformer.

The method described therein works with both electrical switches, IGBTs, and mechanical contacts. It is designed so that the actual load switching takes place in zero-pass of the load current with two IGBTs with diodes in grid switching. An essential component of this known method is the detection and recording of the respective zero-pass as a prerequisite for the initiation of the load switching at that time.

WO 97/05536 provides a further method with an IGBT switchgear, in which the control winding of a power transformer is connected to a common load line by a series circuit of two IGBTs, using the principle of pulse width modulation, and in a further step the current is limited by the transient reactive reactance (TER) of the step winding.

This method requires a specific adaptation of the stepper to the respective stepper to be switched on. In other words, the stepper and the stepper are coordinated and electrically interact. This known method is therefore not suitable for use in a separate, universally applicable stepper not tailored to a specific transformer.

WO97/05536 reveals a method for uninterrupted switching between winding sockets of a step-up transformer with two load branches, each of which contains a serial circuit of two IGBTs switched in opposite directions and a diode is connected parallel to each IGBT.

The purpose of the invention is to specify a process of the type described at the outset which is simple in construction, has a high level of functionality and does not require switching only at zero current of the load.

This task is solved by a procedure having the characteristics of a first patent claim.

This task is further solved by a modified procedure with the characteristics of the secondary third claim.

The methods of the invention are based on the general idea of the invention, namely, not to use varistors, as has been known for a long time from the state of the art, as components for the protection against surge, but to use them for the commutation of the load current of the stepper switch from one side to the other, i.e. from the previously switched winding socket to the new winding socket to be switched, by means of appropriate steps.

In the methods of the present invention, the specially dimensioned varistors connected parallel to each IGBT perform a new function: after commutation of the load current from the switching IGBT to the parallel varistor (small commutation circuit) the varistor passed through the load current builds up a voltage according to its I-U characteristic, which shows a relatively small dependence on the current current value and remains practically constant during the switching operation of the OLTC.

The varistor shall be sized so that the varistor voltage, which results when loaded at the peak of the maximum current, still has a sufficient safety distance to the maximum locking voltage of the IGBTs.

On the other hand, the clamping voltage of the varistors (UVar at 1 mA) must be well above the peak of the maximum step voltage in order for the load current to commute from the switching off OLTC side over the step voltage to the load current receiving side (large circuit of commutation).

The difference ΔU between the moment value of the voltage drop at the varistor and the moment value of the step voltage causes the commutation of the load current via the scatter inductance of the step winding and the line inductances to the receiving side of the step switch and determines the di/dt of the commutation process (ΔU = LKom • di/dt).

It is apparent that the varistors in the present invention do not have the effect of reducing transient surges as is known from the state of the art. Taking over the load current from the hard-shut IGBTsGenerating a voltage drop that, regardless of the momentary value of the load current in a voltage band, must be between the maximum locking voltage of the IGBTs and the peak value of the maximum step voltageProviding a voltage time zone that commutes the load current from the current-carrying side of the switch to the opposite step voltage on the receiving side of the switch:

The provision of the functions listed above by the varistors simplifies and relieves the power electronic switching process in a crucial way:very low energy input into the hard switching IGBTs. Other This fact allows a very simple and cost-effective dimensioning of the power electronic switching groups because the energy-absorbing volume in the case of the varistor is flexibly variable and unevenly larger than the smaller, more expensive and much more variable volume of the IGBT chip.As a further positive factor of the current conduction by the varistor, the availability of the required load-switching time is very important, since the varistor group also allows for a large voltage-displacement and a large tolerance of the IGBT band in terms of the load-switching time.

Fading

The power flow time of the load current through one of the two varistors of the shut-off side is correspondingly extended.

simultaneously

The IGBT groups are switched on and off simultaneously, and no additional load current load times are required for the varistor.

Overlapping

The switching on of the transferring phase switch side occurs before the switching off of the switching off side. During the overlap time, both IGBT groups are closed, so that the phase voltage in this period begins to build up a circulation current. The di/dt of the circulation current being formed depends on the moment value of the phase voltage in the overlap period and the circuit inductance of the circulation current.

The methods of the invention have a number of advantages over the state of the art:

The lowest losses and shortest switching times are achieved when both IGBT groups are switched on and off simultaneously.

If over the years of operation, due to component ageing and workpoint shifting, overlapping or gaping switching behaviour in the control electronics should occur on the order of ± 10 μs, this will not result in a functional hazard to the switch design of the invention.

The only consequences are moderately increased commutation losses and a slightly longer commutation time. In all the three modes of switching described above, the ohmic/resistive energy absorption of the varistors produces an excellent damping of the current and voltage currents during the switching operation as an important positive side effect. Disturbing oscillations, which were expected during such fast switching operations (of the order of 10 μs) in conjunction with the winding capacities and scatter inductance of the step winding itself, cannot be generated due to the strong damping effect of the varistors. In addition, the voltage generated at the varistors as a result of the load current flow is relatively constant and therefore generates a constant diode/zero current. This measure leads to a drastic reduction in the current load of IGBTs and varistors and switching energy losses and a reduction in switching time. Switching to near-zero current allows a significant increase in the rated power output of the stepper switch with unchanged hardware of the power electronic components.

The procedures are explained in more detail below by means of illustrations.

It shows: Figure 1 a schematic of the first process according to the inventionFigure 2 a first circuit particularly suited to the implementation of the process with IGBTs and varistors connected in parallel to each IGBTFigure 3 a further modified circuit to implement the processFigure 4 a schematic of a second simplified process according to the invention

Figure 1 shows a schematic layout of a first process according to the present invention. The method assumes that a step switch, in which a step transformer is to be switched from a previous winding connection to a new winding connection, provides for two load branches connected via a mechanical switch D, DSb and a series circuit arranged in series to it, each consisting of two oppositely switched IGBTs Ian, Iap; Ibn, Ibp with a parallel diode dan, dap; d, dSab, dp, each electrically connected to a common load cell and that parallel to each of the above-mentioned IGBTs a Varistor, Van, Vapbn, Vbn, Vb is connected. Each of the two is to be switched by a load contact MC, or MCa, respectively.

The first step is to fire the mechanical switches DSa and DSb on both sides, which act as the unlock contacts.

Next, ignition voltage is applied to the gates of the IGBT's Ian, Iap of the shut-off side.

The permanent main MCa contact of the switch-off side is then opened.

Once again, the load current IL is then switched to the IGBTs on the switch side.

These IGBTs of the shutdown side Ian, Iap now receive a shutdown command, the IGBTs of the turn on side Ibn, Ibp, on the other hand, an on command.

Err1:Expecting ',' delimiter: line 1 column 112 (char 111)

The load current is now commutated to the van and vap varistors of the switch side according to the invention.

The load current is then commuted to the IGBTs of the receiving side to be switched on Ibn, Ibp.

Once again, the main permanent contact MCb of the acquiring party is closed.

The IGBTs Ibn and Ibp of the receiving side are then switched to the non-conductive state.

The final step is to open the mechanical contacts DSa and DSb, which protect the IGBTs from transient voltage loads that may be applied at the step winding.

Figure 2 shows a circuit particularly suitable for the implementation of the procedure described in Figure 1. Each of the two winding taps tap n and tap n+1 is connected to the step switch via a mechanical switch DSa or DSb, with a series of two IGBTs Ian and Iap on side n and Ibn and Ibp on side n+1 connected to the step switch.

In parallel with each IGBT, a Varistor Van, Vap or Vbn, Vbp is provided.

Finally, the permanent main contacts on each side of the MCa or MCb, which each bridge the entire switchgear in stationary operation, are shown.

The varistor Van, Vap or Vbn, Vbp shall be so dimensioned that its varistor voltage is less than the maximum locking voltage of the respective parallel IGBT but greater than the maximum moment value of the step voltage.

The following is a further explanation of the method of the invention, i.e. a switch sequence from, for example, tap n to tap n+1, using this circuit: in the base position, the load current flows through the main permanent contact MCa from tap n to the step switch drain Y.

The first step in the switching sequence is to close the DSa and DSb switching contacts.

The IGBTs Ian and Iap are then connected to the main DC connector MCa, which switches the load current IL to the IGBT group Ian/Iap.

After less than 10 ms of current flow time from IL through the IGBT group Ian/Iap, these IGBTs receive a shut-off command and the IGBT group Ibn/lbp a simultaneous (at least in the default case) on-off command.

The voltage buildup at the IGBT is transferred to the parallel varistor. When the varistor's clamping voltage is reached after a few 100 ns, the varistor begins to conduct and the voltage at the IGBT splits into two components: The L/di/dt of the small switching circuit between IGBT and parallel varistor.

Due to the very low inductance of the varistor's coupling to the IGBT, the maximum load current from the IGBT to the varistor is switched within 0,1 to 1 μs.

The varistor shall be so dimensioned that the voltage of the load current-carrying varistor is below the maximum locking voltage of the parallel IGBT on the one hand and above the maximum step voltage on the other.

The excess of the moment value of the varistor voltage over the moment value of the step voltage causes the load current to be decommuted with approximately constant di/dt from side A and a shift over the step voltage and the scattering inductance of the step winding Lσ (large commutation circuit) with equal di/dt (in this case positive) to side B. Despite the continuously decreasing current flowing through the varistor on side A, the next voltage remains constant in the first restoration.

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

The varistor voltage breaks down, the transient one. The IGBT naristor group A is equipped with a voltage controller, which is connected to the IGBT and the parallel varistor.

Less than 10 ms after the power electronic commutation of the load current from side A to side B, the DC connector MCb closes and shunts the IGBT group B. The IGBTs Ibn/Ibp are then switched to the non-conductive state via the gate control.

The switching sequence ends with the opening of the mechanical switch-off contacts DSa and DSb, which protect the IGBTs from transient voltage loads that may be applied at the step winding.

Figure 3 shows a modified suitable circuit for performing the process described in claim 1, where the two varistors on one side of Van, Vap or Vbn or Vbp are combined into a common varistor Va or Vb, respectively, with the respective mechanical switch on each side DSa or DSb and the respective varistor on the associated side Va or Vb also forming a series circuit to the common load line.

In Figure 4 a further modified method of the invention is shown, based on a simplification of the process and without any mechanical switches, and the general inventive idea of using varistors to commute the load current is also realized in this method.

This further method assumes that a step switch has two load branches, each of which contains a series circuit of two IGBTs Ian, Iap; Ibn. Ibp, each of which is connected to a diode dan, dap; dbn, dbp.

At the start of the switch, the IGBTs on the shut-off side Ian and Iap carry the load current.

Err1:Expecting ',' delimiter: line 1 column 213 (char 212)

The load current is then switched to and driven by the IGBTs of the receiving side Ibn and Ibp.

As already explained, this simplified procedure is based on a step-change switch which has no mechanical disconnection contacts and no mechanical permanent main contacts, but where the load current is directed by the IGBTs in stationary operation.

Both methods, the one shown in Figure 1 and the one shown in Figure 4, follow the same inventive step and solve the problem of the invention in the same way.

Finally, the advantages of the method according to the invention over the state of the art, which have already been explained in detail above, are to be summarised. Option to switch at any moment of load current without thermal overloading of the IGBTsExtremely fast switching of load current from stepper side A → B or B → A within about 10 μs.Elimination of disturbance oscillationsA specific order adjustment of each stepper to the specific nominal step data of the order case (step voltage, nominal current, scatter inductance) is not possible as long as the limit values of step voltage and nominal current are not exceeded.No robust, self-sufficient switching concept with a very large after-test time range in relation to the write time between the two groups of IGBT switches.Running switches after long operation is required.

Claims (3)

1. Method for uninterrupted changeover between winding taps of a tapped transformer with two load branches (tap n, tap n+1), wherein each of the two load branches (tap n, tap n+1) is connectible with a common load output line by way of a mechanical switch (DS_a, DS_b) and a series circuit, which is arranged in series therewith, consisting of two oppositely connected IGBTs (I_an, I_ap; I_bn, I_bp), wherein a diode (d_an, d_ap; d_bn, d_bp) is provided in parallel with each IGBT (I_an, I_ap; I_bn, I_bp), wherein a varistor (V_an, V_ap; V_bn, V_bp) is provided in parallel with each IGBT (I_an, I_ap; I_bn, I_bp) and wherein each of the two load branches (tap n, tap n+1) can be bridged over by a mechanical latching main contact (MC_a, MC_b), characterised by the following method steps:

   - closing the free-switching contacts DS_a, DS_b of the two sides,

   - applying ignition voltage to the gates of the IGBTs I_an, I_ap of the side switching off and thus switching on those IGBTs,

   - opening the latching main contact MC_a of the side switching off,

   - commutating the load current I_L to the IGBTs of the side switching off,

   - switching off the IGBTs I_an, I_ap of the side switching off and switching on the IGBTs I_bn, I_bp of the side, which is being switched to, in such a manner that the IGBTs I_an, I_ap of the side switching off switch off 'hard',

   - the load current is subsequently commutated to the varistors V_an, V_ap of the side switching off,

   - the load current is further subsequently commutated to the IGBTs I_bn, I_bp of the side taking over,

   - closing the latching main contact MC_b of the side taking over,

   - switching off the IGBTs I_bn and I_bp of the side taking over and

   - opening the mechanical contacts DS_a and DS_b of the two sides.
2. Method according to claim 1, characterised in that in addition a current zero transition detection is carried out and the changeover or commutating process takes place in time proximity to the current zero transition of the load current.
3. Method for uninterrupted changeover between winding taps of a tapped transformer with two load branches (tap n, tap n+1), wherein each of the two load branches (tap n, tap n+1) contains a series circuit consisting of two oppositely connected IGBTs (I_an, I_ap; I_bn, I_bp), 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) and wherein a varistor (V_an, V_ap; V_bn, V_bp) is connected in parallel with each IGBT (I_an, I_ap; I_bn, I_bp), characterised by the following method steps:

   - conducting the load current initially through the IGBTs I_an and I_ap of the side switching off,

   - subsequent switching off of the IGBTs of the side switching off and switching on of the IGBTs I_bn and I_bp of the side switching on in such a manner that the IGBTs of the side switching off switch off 'hard',

   - subsequent commutation of the load current to the varistors V_an and V_ap of the side switching off and

   - further subsequent commutation of the load current to the IGBTs of the side taking over and conducting the load current through these.