# DVR Failure due voltage fluctuations



## nolabama (Oct 3, 2007)

Subscribed


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## Southeast Power (Jan 18, 2009)

Sorry, not really an answer tot he question but why is the voltage fluctuation so high? You wrote 460volts to 600 volts.
Is this due to the voltage regulators not being able to keep up with the load?

Also,
Just for the sake of understanding your system. Does much of the glycol get lost in this process? It reads like it is an open system.


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## varmit (Apr 19, 2009)

Is your power system large enough to handle the heating load in addition to all of the other loads? Without further details, it sounds as if when the SCRs fire, the voltage droops. I would check the amp draw. I am assuming this is powered from a local generator? To run an SCR controlled load this large from a generator would require the generator to be oversized a bit. Also, the SCR cycle times are probably faster than the governor response time of the generator. Could there be a short in the heaters?

If none of the other situations are the problem, an isolation transformer installed in the power circuit, to the heater SCRs, could help prevent generator damage from the high speed switching of the SCR.

Make sure that you are using a good grade of true RMS meter to measure the voltage. Also, check the frequency, on the power system with the heaters off and again with them on. If you have multiple high speed switching devices- VFDs, SCRs and such, they can reflect frequencies back into the power system that can damage other electronic components.


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## wildleg (Apr 12, 2009)

put the critical digital components on a subsystem with a UPS ?


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## JRaef (Mar 23, 2009)

Sounds like you are using an SCR controller that is doing "phase angle
control. I tried to find an "SCR2000" reference to a power controller, I could not so I don't know. But from the sounds of your troubles, it appears to be a likely possibility.

In the Phase Angle control method, there are issues with line noise caused by the SCR firing that is known to affect AVRs. I have not heard of it affecting DVRs because that issue was already well known when DVRs came into existence, so one would imagine that anyone developing a DVR would avoid the potential pitfalls. Most newer AVRs are already designed to not have this problem, but years ago when it first showed up, the solution was to add filtering to the voltage sensing circuit for the AVR. Cat in particular used to sell a retrofit filter unit for a while until they designed it into their production systems. Is that Cat generator older than about 15 years? It's been at least that long soince that problem was prominent.


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## Suren (Mar 12, 2012)

*jrannis*- Mate, we have no answer for why the voltage fluctuates very high. And fluctuations happens only when the glycol system put online.
And that is not immediate. It happened after about 1-1.5 days of glycol system put online. Secondly, we dont have any loss of glycol. In fact, we have tested the generator up to 1.2MWatts during the load test of the generators, our generators are capable of producing 1.2MWatts for continuou operations and the total load is only 50-60% of the continuous power from the generator.

*varmit*- Mate, our generator is 1.5MWatts in capacity and tested for 1.2MWatts for continuous operations. Fluctuations happens only when the glycol system put online. And that is not immediate. It happened after about 1-1.5 days of glycol system put online. We have tested the heater
for short and all the filed power cables were meggered for short and we found no shorts in the system. The current was stable @ 320Amps during the normal condition and voltage fluctuations. Furthermore, the frequency of the engine was at 60Hz during voltage fluctuation and the frequency at DVR was recorded at 50Hz. We are operating at 60Hz. But during operations without glycol system online, everything is fine. Is this due to harmonic distortions produced by the SCR's? Is a linear reactor/line reactor could be a solution for this problem? 

*JRaef*- We have 2 sets of different generator systems. The first one is CAT3512 with CAT SR4 generator system which produces 1MWatts. The second one is MTU-DDC with Marathon Electric generators system produces 1.5MWatts max and 1.2MWatts continuous power. Both generators used with and without glycol system online. Both generators lost their respective AVR and DVR when the glycol system put online. But both generators are more than 15 years old.


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## Suren (Mar 12, 2012)

Jraef- Mate, we have found out that the SCR2000 doing Single Cycle 
Firing technique. Our SCR2000 is from Caledon Controls. 
It is controlled by CB17 single cycle controller and LSC02 load 
sequence controller.


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## Suren (Mar 12, 2012)

wildleg said:


> put the critical digital components on a subsystem with a UPS ?


 
Yes.


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## Dennis Alwon (May 9, 2009)

bump


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## JRaef (Mar 23, 2009)

Suren said:


> Jraef- Mate, we have found out that the SCR2000 doing Single Cycle
> Firing technique. Our SCR2000 is from Caledon Controls.
> It is controlled by CB17 single cycle controller and LSC02 load
> sequence controller.


Forget the CB17 controller, that's not likely the issue.

From looking at the PDF on the SCR2000, this is a zero-cross burst control SCR controller, so that is actually good.

The bad news is, you have a 2-line controller, which is the type that is used to control 3 phase loads by only doing the zero-cross burst control with 2 of the 3 phases. While perfectly valid as a control method, it is not recommended for use on systems with small (relative to utility) local generators because the power draw on the 3 phases is inherent unbalanced and that can also cause havoc in the AVRs. Couple that with the LSC02 that is moving the control around on the output side (as I interpret it), and this looks like it would be difficult for a non-filtered AVR or DVR sensing circuit to handle.

Read this from your controller mfr., it doesn't specifically tell you not to use it on small generator systems, but if you look at the 2-line wave form capture and read the harmonic analysis they did on a 2-line control, you can see where the problem may lie.

You might want to try using a 3 phase 3 line SCR controller instead. You also might want to first speak to your CAT and Marathon reps and ask if they have an AVR/DVR input filter retrofit kit, your's might be old enough that they pre-dated the built-in solution they all provide now.


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## Suren (Mar 12, 2012)

JRaef said:


> Forget the CB17 controller, that's not likely the issue.
> 
> From looking at the PDF on the SCR2000, this is a zero-cross burst control SCR controller, so that is actually good.
> 
> ...


JReaf- Sorry for the wrong information provided. The LSC02 is installed but not activated. It is only installed as precaution for a future use. Sorry again.


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## JRaef (Mar 23, 2009)

Suren said:


> JReaf- Sorry for the wrong information provided. The LSC02 is installed but not activated. It is only installed as precaution for a future use. Sorry again.


It's sTill a 2-line controller so I still have the concern.


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## Suren (Mar 12, 2012)

JRaef said:


> It's sTill a 2-line controller so I still have the concern.


Mate, this is the reply from the makers of the thyristor system:

I have been made aware of this problem by Zoneta, and had sent them advice regarding the firing modes of the thyristor.

Our CB17 controller fires the thyristors in either single cycle burst fire mode or standard burst fire mode. Burst fire mode is a ‘full cycle’ mode of operation – ie both switch on and switch off of the sine waves occur at zero voltage. Single cycle burst fire mode is simply a fast form of burst fire, where at 50% power 1 cycle conducting is followed by 1 cycle non conducting.

Burst fire control is the standard method of control used with resistive loads, as because it does not ‘chop’ the supply cycles the level of harmonic generation is very low. It does however give rise to voltage flicker, and obviously the full load current is being switched on and off at a rate determined by the burst fire cycle time.

Single cycle burst fire was originally developed for use with very fast heaters (infrared etc), but has also been used for many years with generator fed systems. The reason for its use is that burst fire control had a potential to upset the generator speed control which would react to the load being switched on and off and could cause a resonance with the time constants in the speed control. With single cycle control the switching speed is normally too fast for the generator speed control to attempt to follow it. Traditionally generators used electromechanical governors for the speed control, but increasingly, electronic units are being fitted, which probably respond more quickly, and I suspect this is the root of your problem. I have spoken with one of our other clients (a major systems provider) about this. They normally use the single cycle mode of control, but on occasion have reverted to burst fire where there have been issues. In this case you should start with a longer burst fire period, and probably will not be able to make it much less than 5 seconds. I sent some suggestions regarding this to Zoneta. The problem that you have sounds to be particularly severe, and I am not clear why, but is virtually certain to be due to some interaction between the thyristor switching frequency and the time constants in the generator speed control unit.

Because the mode of control involves switching of full supply cycles there is very little difference between 2-leg control and 3-leg control. If you disconnect 2 legs of a 3-phase 3-wire system no current can flow, so it is perfectly possible to switch the current on and off in this way. With burst fire control there is virtually no difference in the current waveforms. With single cycle burst fire control there is a difference which is most noticeable when you are running at half power –(approximately 1 cycle on followed by one cycle off). The waveform on the switched legs is different from the waveform on the unswitched leg, and if the controller is not synchronised correctly the load becomes unbalanced. The current waveforms with correct synchronisation are shown on page 8 of the CB17-2 data sheet. I am not aware of any particular problems associated with 2-leg control as opposed to 3-leg control. The attached document provides some further information on the waveshapes and harmonics. With burst fire control in a 3-wire system the waveforms at start and end of the burst are not pure sine waves. This is because of the 120 degree separation of the 3 phases. At the start of the burst conduction starts between 2 legs (when the voltage across them is zero) and is followed by conduction on the third leg. The reverse process takes place at the end of the burst.

Because the mode of control causes discontinuous current I do not think that harmonics per se are the cause of the problem. Fitting a line reactor is unlikely to have any benefit, and in fact might lead to malfunction of the thyristor trigger circuits, as these are designed for operation with resistive loads in which the voltage and current are in phase with each other.

The other mode of control of thyristors is phase angle (cycle chopping). The CB17 controller does not implement this mode of firing, and it cannot be used with 2 leg control. Because it chops the individual supply cycles this mode of control causes high levels of harmonic current, but conduction is continuous in the sense that there is conduction on every cycle of the supply at all power levels. This method of control has also been used successfully in the past with generators, but we have a client who manufactures non ferrous metal melting furnaces which use this form of control. Until recently operation with standby generators (using electromechanical governors) was problem free, but electronic speed controllers have caused problems, resulting in the generator shutting down. The harmonics created by phase angle control can be removed by filters, but because of the high power levels involved the filters are very expensive.

What do you think?

Rgds,
Suren


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