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Posted on 06 November 2019

Quality Improvement of Reverse Recovery Characteristics

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Measurement of High Voltage Thyristors for Series Connection Application

Nowadays high voltage thyristor stacks based on series connected assemblies are widely used in soft starters for high power motors and high voltage controlled rectifiers in industrial applications and transportation, in HVDC line converter units for the electric power industry. Thyristors in such series connected stacks have some special requirements, the most important of which is the possibility for all thyristors in stack to operate in simultaneity during switching-on and reverse recovery process.

By Vetrov I.Yu., Pallaev R.B., Poleschuk A.V., Presnyakov D.A., Stavtsev A.V., Surma A.M.

Development of a special equipment for testing of high voltage thyristors (and series connected assemblies) in the reverse recovery modes close to typical ones in operating hardware is quite relevant as well. Further, we describe such equipment used at Proton-Electrotex for testing of thyristors adapted for series connection.

System for precision measurement of reverse recovery characteristics of high voltage thyristors and series connected assemblies on their basis

The basic diagram of the testing equipment power part is shown on Figure 1 and includes the following master units.

Basic diagram of testing equipment power part and Unified cell diagram of controlled voltage source

Controlled voltage source unit. The Unit contains a set of series connected consistent cells. An output voltage of each cell Vcell can be set in the range 0500V with the 100V interval. When the unit is on, each cell can generate three voltage levels - «+Vcell»; «0»; «-Vcell», the voltage levels switch at a desired time.

Removable inductive coils unit - This unit enables to choose among five variants of a test loop inductance, which makes possible to vary the rate of anode current in wide range during switching-on and reverse recovery of the thyristor under test.

Reference thyristors unit - This unit, as shown below, ensures precise selecting of the tested thyristors in the groups with low spread of reverse recovery characteristics suited for mounting in series assemblies.

During the testing of separate high voltage thyristors or series connected couples as a part of the controlled voltage source, up to six cells are being switched on, which enables to run tests under conditions close to the nominal voltage and the maximum current load of thyristor. During the tests with high reverse voltage, the thyristors can be connected with damping RC circuits.

The unit can be used for thyristor stack testing with voltage in the range 6-10kV, for which quantity of cells in the controlled voltage source can be increased.

A Unit control system enables to generate five time intervals (phases) with controlled length, during each of those the voltage source cells are enabled with certain voltage level as it is shown on Figure 2. During the test of reverse recovery characteristics, the following phases are usually considered.

Control system window for testing mode selection

Phase 1: Increase of tested thyristor anode current with controlled rate of rise. Rate of rise depends on the total positive voltage of the controlled source cells and the selected inductive coil.

Phase 2: Maintenance of anode current on a desired level. The controlled source cells operate in the pulse-width modulation (PWM) mode which enables to maintain required anode current with a small oscillation against the given level.

Phase 3: Stabilization of anode current prior to reverse recovery process. The total voltage of source cells has low positive or zero value, that ensures the change of anode current with low speed to make precise level of anode current, which commences reverse recovery process.

Phase 4: Reverse recovery process. The total voltage of source cells has negative value corresponding to given value VRDC for the tested thyristor. Controlled rate of anode current fall is defined by the given inductive coil.

Phase 5: Second applying of direct voltage with required peak value to the tested thyristor. This phase is handled to prove meeting the requirements for turn-off time (tq) standard of the tested thyristor.

During the test, the computerized system collects and processes the sensor signals of thyristor anode current and voltage and displays waveforms of current and voltage along with numerical values of the main characteristics: reverse recovery time (trr) and its phases (ts и tf), reverse recovery surge current (IrrM), approximated (Qrra) and integrated (Qrri) reverse recovery charges (Figure3).

Data collection and processing system window

The typical statistical distribution of reverse recovery charge for high voltage thyristors adapted for series connection is shown on Figure 4a. RMS deviation value Qrr normalized to the mean equals about 2.5% in big lots of such thyristors. Herewith the contribution of measurement accuracy and actual variation of Qrr in measured value defined by controllability of technological process is approximately the same.

Typical statistical distribution of reverse recovery charge in lots of high voltage thyristors

It is possible to avoid the impact of measurement error and organize more precise group selection for series connected assemblies by parallel testing with reference thyristors. This method is based on high sensitivity of anode voltage distribution in the reverse recovery process of couple series connected thyristors to the difference of Qrr values for these devices. Thus, typical dependence of maximum pulse voltage VrrM variation, which occurs during the reverse recovery of couple series connected thyristors as a result of difference in the reverse recovery charges is shown on Figure 4b.

Typical dependence of maximum pulse voltage VrrM variation during reverse recovery of the couple series connected thyristors as a result of difference in reverse recovery charges

It is clear that in the case of low values of reverse recovery charge difference this dependence is close to linear. This enables, using reference thyristors with Qrr values gradually increasing by 2-3%, to group the tested thyristors by this characteristic with precision higher than 0.5%. Such grouping precision enables to obtain simultaneous reverse recovery of the thyristors in series connected assemblies even without snubber RC circuits. As an example, recovery of the couple of high voltage thyristors T653-630-65 (average current 630A, voltage 6500V) is shown on Figure 5.

Simultaneous reverse recovery of the couple series connected high voltage thyristors anode current and voltage waveforms

Conclusion

Application of measurement equipment complex described above in production of high voltage thyristors adapted for series connection enables to improve quality and reliability of high voltage thyristor switches based on series assemblies for power electronics in industrial applications, transportation and the electric power industry.

 

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