six pulse bridge rectifier

I am having problems measuring the voltage of a 600 kW six pulse bridge rectifier with a simple multimeter .... it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs typical time) the AC burs after a Current transformer (???) attached to each one passes current to thyristors in pair When I power ON at around 250 kW , it is an induction furnace solid state inverter at 600 kW - yet I am reaching just the 500 kW for the moment ... and I measure (multimeter in AC V scale) the AC bars in one end and the other to DC (upper thyristors blocks) I get respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!), to the lower thyristor block from AC bars again , low voltage like

120-220 Volts DC+DC- around 1000 Volts when power of the generator is OFF , normal 400 V AC is coming through the 3 AC bars .... measuring the AC burs at the exit of the main switch from my board to the cabinet I get 500, 730, 840 Volts respectively the AC bars leading to the thyristor pairs usualy overheats, I suspect it is a thyristor triggering related problem but cant measure them correct , though in 200 Ohms none is showing short circuit

thyristors are like ...

1 3 5 2 6 2

and bus bars 1 2 3

I would apreciate if someone could enlighten me on how to measure the voltages and/or thyristors thanks !

reply:

It is "electronics" in the title of this NG

What's a "six pulse bridge rectifier" ? Do you mean three phase by any chance ?

Graham

Holy crap! You are playing with 1800 volt 3-phase AC at 600 KW power levels, and you don't know how to run a multimeter? Where shall I send the sincere condolences to your wife?

Bill

=CE=BC=CE=AD=CE=BD=CE=BF=CF=85 =CF=83=CE=B5 =CF=80=CE=B1=CF=81=CE=AC=CE=B8= =CE=B5=CF=83=CE=B7 -

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lol Bill :D she will be happy if I fly away :) .. it is a malfunction I have to resolve, service is not very helpfull and I am not an electrician Though from practice I know some things ... but I get puzzled on the basics :D If you do know how a bridge full wave rectifier works you can tell me how to be sure that I measure the correct voltage DC or AC ...

six pulse bridge rectifier to my knowledge is a device consisting of two thyristors for each AC phase. 6 in total ... with the correct triggering they convert the AC to DC one way always ... 3 pairs , one pair for each AC phase ... current runs one way only throug two of the thyristors for some mseconds adding DC voltage ...

you are dealing with a 3 phase, phase firing system. if you need accurate measuring. You need a voltage isolator+ divider and a scope to monitor the output..

And for your problem that you're trying to detect, you either have a miss aligned firing drivers, bad thyristors or shorted turns in the load that is changing the induction.

Connect the load into a 3 phase Resistor network, take the measurements from there and get your self a scope!... It sounds like you need to tailor the firing thresholds.. Using a Reactor may also help out.

--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

Thanx Jammie

I shall check them all ... I suspect a thyristor that is not triggering right ... propably on the pair I get the low voltage readings ...or the high ones ...lol... and I willl find me a scope to see the wave coming out to DC , I think I know the pins to measure it externaly

The generator is working for two melting furnances so it is evil coincidence to have a short circuit at both at the same time ... but you never know

thanks for the advice

You are seriously over your head here and should not be playing around.

A multimeter is not the correct tool for this job!

You need

3-4 channel scope with triggering capability 4pcs. X10 probes 4pcs 4kv probes 3pcs 3000A Dc/high frequency current probes.

480V to 24 V control transformer

4pcs. 1500v wideband isolation probes

In any case, you are not even measuring the voltage correctly.

The incoming AC is measured:

buss to buss ie Black - Red ie Red - Blue ie Blue

- Black (DELTA or WYE)

or buss to neutral. ie Black - White ie Red - White ie Blue - White (WYE only)

or buss to ground ie Black - Green ie Red - Green ie Blue - Green (WYE only)

The main 3 phase thyristor bridge you describe should not be making DC, it should be making high frequency AC, hence the description inverter! I would assume, the multimeter measurements would be consistent from pair to pair but totally inaccurate- you will have no idea what the true voltage or waveform really is.

Now there are 3 input AC busses, each connected to a pair of thyristors, one positive, one negative. On the output of the thyristor set, are there just 2 output busses or are there 6 and do they connect directly to the load? Is the load connected to ground or neutral in anyway, perhaps in the center of the coils?

This whole thing sounds really dangerous at best. The AC supply side should be stable +/- 15% with the thing on or off and in no case should a 440vac phase start reading 600v or 1000v to ground at the AC Distribution (Circuit Breaker or Fuse) Box. If it does, you've got big problems and they might be utility related. An installation of this size will usually have a company supplied step-down transformer from say 13.5KV to 440 made up of 3 interconnected transformers one for each phase. If the junction that connects the three secondaries becomes weak, that is, fails under load, the system goes up for grabs. That is in a WYE connected system. In a Delta connected system it is yet....Well here you read about it:

Since the secondary of the Delta is not ground referenced, measurements that are ground referenced or neutral referenced have no meaning and can vary wildly. You must measure Buss to Buss, Phase to Phase. I am guessing you have a Delta Connected distribution transformer for this inverter.

With the inverter off at the input side:

Trigger the scope with a 440v to 24v control transformer. Connect the

440v side from phase A to B and ground one leg of the 24v secondary. Connect the X10 probe to the other 24v leg and the probes ground where you grounded the other side of the secondary. Connect the probe to the trigger input. Set the scope for external triggering and adjust for a stable trigger at a 60hz rate. ( You can cheat here and use the LINE TRIGGER setting instead)

Using the 4kv X100 probes, Connect both probe grounds to the ground used above. Set the Ch1 and CH2 scope inputs to add. Press the invert button on CH2.

1) Connect one to A and one to B.

Set both CH1 and CH2 to identical attenuator settings in the CAL position so the trace fills the screen vertically (about 1-2 V per division). Adjust the sweep so you can see 2 complete sine waves.

2) Move the probe on phase A to C and measure the B - C waveform

3) Then move the probe on phase B to A and measure the A - C Waveform

All three should be about the same

Connect your current probe to CH3 and set the attenuator for about

1500A full screen using the probe factor, DC coupled .

Start the inverter

4) Clamp it around each of the 6 thyristors or it's output lead, one at a time noting the CH3 waveform

5) Clamp it around each of the 3 AC input busses and note the CH3 waveform

Repeat 1) 2) and 3) with the inverter still running.

Repeat 1) 2) and 3) with the inverter still running but this time connect to the output side of the thyristors.

Be careful, setup each measurement, use protective gloves, a switchgear mat and have an assistant standby with an 8' 2X4 to pry you off if you get grabbed. You are working at massive power levels. The fact that Busses are overheating indicates a high current fault that could fry your face at any moment. Death by electrocution is very painful. Death by molten copper plasma is excruciating.

At this point the faulty section should be obvious. You could take a chance and replace the offending thyristor.

The next test now that you have narrowed it down to a single device requires you to measure the trigger voltage and this can be tricky since it is .6v (actually it could be 20) referenced to one side of the thyristor in a really noisy environment. Measuring it wrong can trigger the thyristor at the wrong time and then you're in a world of shit. Use 4KV probes on the three terminals of the thyristor. The gate terminal will be pulsed a few volts above one of the other two terminals.

Using the 4kv X100 probes, Connect probe grounds to the ground used above. Set the Ch1 and CH2 scope inputs to add. Press the invert button on CH2.

a) Connect one probe to the GATE and the other to each of the large terminals of the thyristor.

Set both CH1 and CH2 to identical attenuator settings in the CAL position so the trace fills the screen vertically for each arrangement. The correct orientation will require a lot of gain around

1-10mv/Div . Start at 10v/div and reduce 1 click ch1 then 1 click ch2 etc.

Repeat this test on a known working thyristor for reference.

Where are you located? Thought of hiring someone to do this for you?

Tom

Thanks for your informative reply , I owe you I am in Greece and waiting for a specialist to come from England, unfortunatley next week :( so I had to do something myself ...I understand the danger and I will follow your instructions step by step. Trying to work again today since the voltage -not accurate measures I took looked "normal" to me, that is instead of 400 V/50 Hz ... 430-460 V due to harmonics I supose. So after talking to him despit the heating up of the busbars he told me I can work ... I have a short circuit thyristor more :((( Amperage was not high , according to the instruments steadily at 900 A

- it is designed for 1.300 A, thyristors can take 1800 V - Itav 1400 A Yes , it is an Inverter ... I take two bus bars from the 6 bridge rectifier SCR's that is DC ... passing through a secondary protection coil goes to an Inverter ... 2 pairs of fast , inverter grade thyristors and they finaly give to the Load high frequency AC at

600-1000 Hz All started two weeks ago with the secondary coil overheating - just one widing , it has two , one for each DC phase I supose I was told that it is short circuit and it went for rewind ... waste of time and money ... After having the same overheating at the secondary - protection coil ... I was told and it was true that I had one water cooled cable to the load cut ... it was so After that I was able to work for some melts until the source coming bus bars started to overheating ... I though it was from bad thyristors they can;t take too much heat for long , though the system has everything on it like drops the power from overheating at various points etc. Water that cools the cabinet components are highly deionised but I noticed that in this particular winding , water pressure was low .... the same cooling line passes from the thyristors so I am thinking of giving a new one just for the coil and thyristors with enough pressure to cool them down Tomorrow I will have one new slow rectifier phase thyristor to test I am also suspecting short circuit at the load coil ... it is heavily built up with yokes and supporting woods and isulations between them ... so I have to exclude any short circuit conditions on the load side I exchange two thyristors ... 1-3 and the voltage ... with the multimeter I did measured it ... seems normal as I told you before . THanks for the thyristor and triggering instructions I shall try them also tomorrow !

Anybody else out there following this thread with absolute horror? I have been in electronics for 40 years and I am picturing all sorts in my minds eye. One thing is for sure, I wouldnt go within 100 metres of it when its fired up, faulty or not! Gordon

Ha, don't let that bother you, I've had 2500 Amp Scr's explode from old reliance drives right in front of me!, ceramic chips everywhere! :)

Some time's they don't die silently!

--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

=F5 =F3=E5 =F0=E1=F1=DC=E8=E5=F3=E7 -

=E5=F3=E7 -

OMG Jamie ! ... I saw one once getting red and thats all ... now you really scared me

Likewise I went to a regional power station open day one day. It was necessary to kick in one generator set while we were there. I couldn't believe my eyes when someone had to open up a door on the side of the casing, reach in , and do something, I don't know what, and there was an almighty roof rattling bang as bus-bars contacted or something. The public , including me, was 50 yards away. Anyone ever visited the National Grid pylon and insulator test facility at Leatherhead, Surrey , sci-fi kit there.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

If you have to come to a group of strangers for help, you may be asking for it...not the help...an electrical mishap of epic proportions! I'll keep my eyes on the news for Greece being mentioned!

I looked at your schematic and you are right, the first six are rectifiers but I suspect they form a synchronous rectifier with phase control. Their output is pulsed DC and the output voltage is controlled by the on time. since the supply voltage is 120 degrees per phase offset, they are pulsed in sequence BUT each should be triggered with the same time offset from the zero voltage crossing of it's corresponding phase to prevent current hogging.

I cannot tell if what you are calling "protection coils" are in fact noise filters, swinging chokes or saturable reactors. The giant MOV and reverse diode should also be checked.

It looks like there are a triple set of control transformers in the electronics package. There may also be isolation transformers at each of the thyristor trigger modules that could simplify your testing of the upstream components, timing etc at reduced voltage potentials and unified circuit ground, ie if there is 0 ohms between the low side of the trigger transformer primaries, that is all are connected to the circuit ground and that ground is at earth, chassis potential, you can ground your scope probe at that point and probe the high side of the trigger transformer primaries to rule out problems in the circuit board itself.

There is no substitute for an AC/DC current probe in your line of work. It is a must purchase item. At least an ampclamp meter but preferably one for use with a wideband scope. Tektronix makes a very nice one with a 3000A adapter and jaws big enough for your application. The smaller probe can be used on the gate leads. The larger on high power portions of the circuit. They are inherently very safe BUT be aware of the open laminations when clamping the jaws.

Get a captive ball flow meter sight glass and splice it into your cooling lines to verify flow.

Shorts on the load should be evident and in fact dramatic.

What is the status of the high frequency thyristors? I think they would be more likely to toast the "protection coil".

None of these ratings you specify for the thyristors have ANY RELATIONSHIP TO THE WAY YOU ARE MEASURING THEM. What I am trying to say is, all of your power devices have been subject to overloads way beyond design values and must be tested for leakage in the off state, voltage drop under full load, excess heating under load etc. When you connect a scope you will find peaks of current and voltage far beyond what your multimeter says. Those values correspond to semiconductor ratings.

I would MOUNT the scope on this inverter to view both voltage and current waveforms being sent to the furnace along with the existing panel meters. You might be able to use existing meter shunts to drive the scope. This will give you early indication of impending failures as you are using the system.

Tom

Tom

I had a qualified electronic repairman yesterdays ... we did measuree the voltages with a descent multimeter and was all normal ... 390 V phase to phase AC , around 400 VDC all phases equal ...so the indications i took with the cheap multimeter was all wrong. We also been able to measure pulses and seems that all were working, triggering the tyristors Also the wayform of the output AC was acording to the shape of the manufacturer at 1,5 V scale as expected Still the secondary coil ... DC choke actually overheats 2 timesa during a 1 hour melt ... I never power the load at maximum , I work at

60% maximum , takes some time more but works , I also lower the cooling water maximum allowable temperature so not to heat up the thyristors a lot You say that high frequency thyristors ... I meant fast thyristors 60 microseconds instead of the 200 of the rectifier grade thyristors ... may involved and I suspect the same along with load short circuit The power cabinet is connected to two furnaces ... one is working at a time ... and the two have different behavior ... 500 kgs start at 500 A and the 1000 kgs ..with the same metal charge starts at 900 A! so it must have something to do with the load shortcircuit too One expert from Bulgaria is expected today and maybe next week one from England .. .we will see Thanks for the suport my friend, got to go to work now I will study your texts later Rex

As for the cooling system .. it is very sophisticated and complete of floaters , sight eye glass .. a total of 18 pieces .. 6+6 for loads and 6 for the power cabinet itself , the power cabinet floaters have thermoseters so to cut power off due to high temperature , as I told you I have adjust this setting to the lowest possible so to cut off earlier and not overheat too much the power thyristors I have to get me one pair of fast , inverter grade thyristors , the one pair is 1 year old and it was an alternative choice of a russian manufacturer, Westcode has long delivery times .. they are faster at

14 microseconds but a design parameter is kept low so the clamping plates of copper may short circuit around the thyristor and to the extend of the cathode or anode surface ... i saw some silver bubbles there :D so they may have overheated due to this fact and who knows what is happening inside them ... but the load short circuit is still what concerns me ... furnace used to work before with considerable fluctations in all the iondications .. KW, A, V or even frequency .. now it goes steadily increased as if it only one turn to read and adjust ... I mean by this that adding metal charge, filling the furnace does not affect its performance as it did before .. it is said that electronic board adjust to the sload so to have full power always ... this is not happening now I simply raise the power by the power regulator

This is just scary. This guy doesn't even own a decent multimeter or even know how to use one and he is playing with a 3 phase HV inverter! I wouldn't go near this thing either. I hope this guy doesn't end up killing himself. I'd say he should invest in a good life insurance policy.