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|Title: ||Stray inductance effects and protection in GTO thyristor circuits|
|Authors: ||Al-Hakim, Husam A.|
|Keywords: ||Gate turn-off thyristors|
|Issue Date: ||1990|
|Publisher: ||© Husam A. Al-Hakim|
|Abstract: ||The recently developed gate turn-off thyristor is now becoming well established as the
first choice switching device in high power converters for applications such as
uninterruptible power supplies, frequency changers, and AC and some DC variable speed
motor drives. The special operating features of these devices in conventional circuit
configurations are investigated.
The GTO thyristor physical behaviour and operating characteristics are first described
and supported by measurements made at turn-off currents of up to 600A on a specially
constructed test circuit. From this, it is shown that, owing to the extremely fast rates of
fall of anode current at turn-off, voltage overshoot effects caused by the stray circuit
inductances are highly dangerous to the device, and effective snubbing is essential.
A detailed study of these stray inductance effects in constructed DC chopper and H-bridge
inverter circuits follows. The circuits are modelled to include these strays, with
appropriate mathematical analysis and computer simulation, to determine which stray
inductances are the most influential in causing GTO thyristor voltage stress.
The different switching patterns are considered for the H-bridge to provide
quasi-square and various pulse width modulated (PWM) output voltage waveforms, and
the detailed current transfer paths in the various circuit devices and snubber
components defined and mathematically analysed in each case. Practical switching
effects of diode reverse recovery and GTO mismatched switching times are demonstrated
and possible damaging conditions revealed.
All analytical and computed results are supported by experimental measurements.
A GTO thyristor will be damaged by attempting to turn-off an over-current, and
satisfactory protection against this is essential. Conventional fusing is usually
inadequate, and a better method is to use a fast active system utilising either a crowbar
and fuse, or rapid direct gate turn-off. Both methods are investigated and experimental
results provided. It is concluded that, with appropriate circuit layout and component choice, the
unavoidable stray inductance effects can be limited to manageable levels. The most
severe effects are caused by the DC source inductance which is the most difficult to
minimise. Others within the power circuit, if kept small, will have a marginal effect.
Fast over-current protection is achievable|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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