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cafergav
Participant

Join Date: Sep 2013
Posts: 3

Burned Thyristors of AVR - Synchronous Motor

09/21/2013 6:33 PM

Hi Dear all

Our Plant has separate generation and our main loads are four synchronous motors 73000 HP, when we have suffered electrical disturbances on our HV bus always one or two thyristors of the AVR's thyristors bridge has resulted burned.

For along time we though that the cause of burned thyristors was due overvoltage on electrical system.

At present time we bealive that the cause is associated to a very high current, when voltage drops on motor terminals the AVR's response to recover voltage on motor terminal is too agressive, producing damage on thyristors.

Motors are brushless type, and the AVR power supply is taken from the auxiliary services bus.

I've been reading literature about thristors and most comments are that thyristor do not burned by current they resulted damage by voltage.

I've been trying to upload some pictures but it has not been possible the web page give me an error.

Does someone has experimented similar case or has an idea about this failure?

Regards
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powersolutionsFBD
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Join Date: Jul 2013
Location: Faridabad Near New Delhi India
Posts: 240
Good Answers: 34
#1

Re: Burned Thyristors of AVR - Synchronous Motor

09/22/2013 7:26 AM

Two reasons for failure of thyristors.

Over voltage due to sudden switching or out of step synchronizing with grid. Normally MOV varistors are provided across thyristors to protect against over voltage. Other way to protect against frequent failure due to over voltage is to use Thyristor of higher PIV. If you have installed Numerical type Protection Relays in system then you can find in log of relay the range of voltages experienced by equipment during such disturbance.

I agree with you that mostly the reason is not over voltage but over current for such failures.

Reason being that when there is a system disturbance- that means dip in frequency and voltage, both will result in sudden increase in excitation current.

To protect overload of excitation system due to dip in frequency install Volts per Hz characteristic module with AVR. This module automatically lowers the internal voltage set point of AVR proportional to system frequency. This further results in draw from Grid capacitive current for additional excitation to match grid voltage and protects Excitation system from over excitation.

To protect the synchronous alternator from voltage dips - which is more dangerous to excitation system use Automatic Power Factor Limiting Module.

Effect of voltage dip in large size Grid system to which relatively small Synchronous motor/alternator is that, the synchronous alternator tries to maintain grid voltage to set point hence feeds heavy capacitive current in to grid (Inductive current for Synchronous motor). This results in severe overload of excitation system.

Power Factor Limiting or kVAR Limiting Modules are available with various make of AVR. By installing one in each AVR, you can set either Power Factor (say 0.8) or Maximum kVAR limit.

During system disturbance above module will ensure that your motors do not jump in to restore system disturbance, resulting in overloading and failure of Thyristors/ Diodes.

Hope above input could solve your problem.
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67model
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Join Date: Dec 2010
Posts: 1686
Good Answers: 116
#2

Re: Burned Thyristors of AVR - Synchronous Motor

09/22/2013 2:33 PM

Thyristors can be damaged by an excessive rate of rise of current after the initial firing.

Since you write that you have very high currents, it is possible that rate of rise of current exceeds the specified value for the thyristor. It hits the slowest individual, most items exceed spec. As with many failure modes, it is exacerbated if the device is already hot, since failure is caused by melting at a hot spot.

If the maker is no longer there to help, it may be possible to replace thyristors with a type which has the same spec as existing but much faster and higher voltage, even higher current. The other approach is to increase the source inductance, to slow the rate of rise.

Another important factor is the firing circuit, for tough thyristor conditions, rapid rise, high peak voltage and current into a resistive load are essential to get as much initial triggering as possible.

Also R-C snubbers may have degraded, a capacitance check may reveal damage - these stop high dV/dt.

Constructions are seldom symmetrical - have you noticed that failure occurs in certain positions, which might have less inductance or degraded firing circuit??

The semiconductor maker may be able to help with interchangeable improved devices. De-canning and examination of die can isolate the cause of failure. Failure by dI/dt is distinctive if the "follow-on"current is not so high that it destroys the whole die.

Have you checked that the control circuits are actually working to specification and the overload settings are as required for the plant??
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cafergav
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Join Date: Sep 2013
Posts: 3
#3
In reply to #2

Re: Burned Thyristors of AVR - Synchronous Motor

09/23/2013 12:27 AM

In normal operating conditions the bridge operate excellent and it is very reliable. Under system disturbances is not reliable, as you mention failure occurs always in same position additional to damage on thyristors (thyristors' housing used to explode and also the gate, always presence of black powder), the triggered pulse card's gate connection to the affected thyristors used to burned too.

Snubbers did not suffer damage. Bridge has very good ventilation but thyristor are in hot operation so their capacity is limited if there are exposed to sudden variation di/dt.

I been trying to get support with the maker but has not been successful answers
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67model
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#4
In reply to #3

Re: Burned Thyristors of AVR - Synchronous Motor

09/23/2013 8:30 AM

If the thyristors are blown apart, I wonder if you have the correct semiconductor type fuses fitted to suite them. The "if the fuse blows, then replace with a bigger one..." solution may have been applied before your arrival.

a) Are the fuses type/voltage/current rating "to drawing"?

b) Have you a note of the damage to trigger pulse cards?

c) Have the trigger cards been replaced - or "repaired, not replaced", after failures? Are there any trigger pulse isolating transformers or is it direct connection?

d) Do you have the thyristor types code numbers? Are thyristors [SCR] in parallel and/or series connection? What is the AVR basic voltage/current rating? How many SCR?

e) To avoid the problems/time repairing "consequential" damage to trigger circuits it may be worthwhile fusing the gate connections with "meter" type fuses. And fitting much higher rated resistors/transistors in place of burned ones at trigger pulse output stage.

f) In the usual way of things, the AVR limits are set to field current/time limits of generator field - these are many seconds. Do you have any record indicating SCR failure is immediate or delayed from start of Grid Disturbance?

67model
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cafergav
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Join Date: Sep 2013
Posts: 3
#5
In reply to #4

Re: Burned Thyristors of AVR - Synchronous Motor

09/23/2013 5:24 PM

Very good observations, my comments:

a) Are the fuses type/voltage/current rating "to drawing"?

  • This circuit does not have fuses. It is protected by shortcircuit with a model case circuit breaker, according to drawings.

b) Have you a note of the damage to trigger pulse cards?

  • Ampl. trigger pulse card always result burned on terminals associated to the gate of the damage thyristor also isolating transformers burns on ampl. pulse card

c) Have the trigger cards been replaced - or "repaired, not replaced", after failures? Are there any trigger pulse isolating transformers or is it direct connection?

  • The ampl. trigger cards are replaced after failure. Yes there are isolating transformers that burns too

d) Do you have the thyristor types code numbers? Are thyristors [SCR] in parallel and/or series connection? What is the AVR basic voltage/current rating? How many SCR?

  • SKKT273/18E Semikron
  • AVR rated voltage 480 Volt, rated current 170 Amp. Bridge is conformed by 3 (SKKT273) one per phase.
  • Install fuse in gate connections could prevent damage on card, but could this affect firing signal?

f) In the usual way of things, the AVR limits are set to field current/time limits of generator field - these are many seconds. Do you have any record indicating SCR failure is immediate or delayed from start of Grid Disturbance?

  • This is something I am trying to determine but I could not do it yet.
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67model
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Posts: 1686
Good Answers: 116
#6
In reply to #5

Re: Burned Thyristors of AVR - Synchronous Motor

09/24/2013 11:05 AM

Thanks for info Cafergav.

"First cut" is...

  1. Semikron make SKKT323 in same housing, appears at first glance to be uprated version with 320amps rating versus 274 amps at case temp 85 'C.
  2. http://www.semikron.com/products/data/cur/assets/SKKT_323_18_E_07890230.pdf.
  3. I2t for SKKT274 is 320000 A2s compared to 50600 [7790 pre-arcing] for 170 amp rms 1000 VRMS 100 kA break SIBA brand semiconductor fuse in 27mm Ø x 60 mm long size.
  4. One has to be sure that the 170 amp AVR rating does not hide big over-currents for field forcing. Do you have any information on AVR field current limit settings, amps/time?
  5. Copper wire 0.2mm diameter has a fusing current of 7amps in a re- wireable fuse. A quick comparison with PVC cable tables S/C capacity [for 130'C rise] suggests 3 amps for 1 second [I2t = 9]. What is the wire size on the SCR side of the trigger isolating transformers?
  6. A 2 amp 1000V 30kA 6.35 x 32 mm SIBA DMI meter fuse has 0.35 ohm resistance and I2t = 6. Do you have a resistance value for SCR side of trigger transfo + wiring?
  7. Subject to check, I would expect trigger circuit to deliver 10 volts 1 amp for hard firing, so 0.35 ohms addition should not be significant.

67model
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