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Anonymous Poster #1

SCR DC Bus AC Leakage

04/04/2016 6:38 AM

In the drilling rigs , they Use SCR system to Convert AC to DC

my query is if i measure the AC voltage on the DC bus how much voltage AC will i get if the input is AC 600 Volts and 60 HZ and all SCRs are good and firing properly

How much frequency will i get at the DC bus if the input is 600 VAC ,60 HZ and all the SCRs are firing properly

Also does a high value of AC voltage measured on the DC bus indicate aging or problem with the SCR , how to diagnose these problems

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Rixter
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#1

Re: SCR DC bus AC leakage

04/04/2016 8:24 AM

There is no simple answer. It depends on the filtering, the SCR firing point, and the type of AC meter you use to measure it.

SCRs control the output DC voltage by firing at some delay after the zero crossing of the AC input waveform. (The control circuit actually controls the firing point of the SCRs.) This sudden voltage transition as the SCR turns on creates a large number of higher harmonic components. The higher the AC waveform is when the SCR fires, the more higher harmonics will be generated.

The amount of rectified AC ripple plus these higher harmonics that is measured on the output depends on the filtering and the frequency response of the AC voltmeter.

I would advise looking at the output with an oscilloscope. (Make sure the scope can handle the output voltage.) Each of the three phases contributes on both positive and negative input, six times per cycle or 360 times per second. Look for uniformity. If one or more of the six "look different", it could indicate a problem with the control circuitry or a component of the filter (likely a capacitor).
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marcot
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#4
In reply to #1

Re: SCR DC bus AC leakage

04/05/2016 9:12 AM

GA it is true that the specifics results may vary.

But I would suggest to use a current probe instead of a voltage measurement as they are isolated and safer to use.

To be more specific, a good troubleshooting tool is a Rogoski coil to measure the "AC" part of the current on your oscilloscope. (If you can afford it, a large DC current probe will be even better).

The Rogoski coils do not stick or saturate when measuring DC. They also come with 20-30 cm dia. loops very convenient for high power equipment.

By wrapping it around each SCR, you will see their respective AC current. They should be all similar. If one SCR is open (or failed gate driver) or short, its waveform will be greatly different from the others.

Another troubleshooting trick if an oscilloscope is not available is to add a dab of non-conductive, mat paint to each SCR and compare their respective operating temperatures. They should also be all similar.

Good luck and be safe! (Lock up (off) and tag before touching...)

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TonyS
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#2

Re: SCR DC Bus AC Leakage

04/04/2016 12:25 PM

Load will have a lot to do with the amount of ripple on the DC bus

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GW
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#3

Re: SCR DC Bus AC Leakage

04/04/2016 11:30 PM

60Hz x 6 legs - fundamental ripple frequency is 360 Hz. And that is rich in harmonics.

Phase angle, filtering, characteristics of load etc. will determine the magnitude and wave shape of the circuit.

With parallel SCR's as shown you will need to test each SCR individually, or at least monitor the current through each SCR. The rectified Voltage waveform may not identify a bad SCR. The Voltage Wave form across each SCR may identify a pair of bad SCR's.

Bottom line - you need to use a scope, and know what voltage / current waveform you are expecting. There are many good books on the topic. I recently saw one on the internet by Juan Dixon "Three-Phase Controlled Rectifiers"
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marcot
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#5
In reply to #3

Re: SCR DC Bus AC Leakage

04/05/2016 9:17 AM

GA.

Good choice of author. I had the honor of working with Dr. Dixon many years ago. Very competent engineer and the nicest man you can find.

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GW
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#6
In reply to #5

Re: SCR DC Bus AC Leakage

04/05/2016 9:57 AM

I was taught by Prof S B Dewan back in the 70's - He was one of the best prof's I ever had. He would be in his 80's if he is still alive. (Dewan and Straughen - Power Semiconductor Circuits)

I try to refresh my knowledge every now and then - and I found Dixon was readable and hit the key points.
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minggrabber
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#7

Re: SCR DC Bus AC Leakage

04/10/2016 12:48 AM

The latest measurements shows that two scrs out of 4 are showing frequency of 360 hz but the rest two are showing 300 and 255 HZ . this indicates there is a problem with the firing of the SCR
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vsar
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#8
In reply to #7

Re: SCR DC Bus AC Leakage

04/10/2016 8:49 AM

It helps to connect resistive loads while troubleshooting, this minimizes unnecessary noise that may be seen in the scope. As shown, Each of the 3-phase inputs are fed in between a pair of SCRs (between the anode and cathode junctions) of what appears as a 2-packaged sets of SCRs for each phase.. The paralleled SCR with an RC seems to be a snubber circuit for each leg's main (bigger) SCRs protection? ..

It seems that each line of the 3-phase AC are being controlled with both the positive and negative cycles for DC conversion, by controlling the firing angles of each Power SCRs..

It it therefore imperative and critical to ascertain the uniformity of the generated frequency of the firing circuitries! They should be the same and synchronized! The power SCRs gate characterisitics is also critical! Verify the 3 positive and 3 negative cycle output waveforms are sequenced sharing a uniform phases and firing angles!

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GW
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#9
In reply to #7

Re: SCR DC Bus AC Leakage

04/10/2016 11:13 AM

Please show us some scope snap shots with the points indicated where you measured.

It will be educational at least.
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