HV DC LINK TRIP IN VFD 1

[Electricrifying]

I am facing problem with my 690V,1500kw vfd unit which is 7 years old(make:Nelco),It is a 12 pulse rectifier unit driving a PD pump(Make:Geho) for dense slurry disposal with output pressure upto 160bar.It has functioned well till three months before the vfd started tripping on HV DC LINK BUS alarm.Incoming voltages has been checked found within limits,The output reactors are also found healthy, On calling the service engineer team, they changed few speed and current loop parameters and the drive started functioning well again but since last one month the tripping has again started (without any change in the load).On checking in oscilloscope we found that at times for afraction of second the rpm suddenly shoots up/down(about30rpm deflection) and the dc voltage at the instant ahoots up above the limit(max dc limit:1100VDC),leading to tripping of the drive. Plz help.

 

[Electricrifying]

The I/P voltage is 694V.

 

[Electricrifying]

Replaced the capacitor bank with new one but the problem still persists, can you give some light on how speed and current loop gain are calculated and the importance of these parameters in vfd...

 

[ScottyUK]

Before assuming the drive is at fault and getting into the detail of programming the loops, start with the underlying problem.

An increase in DC link voltage requires a source of energy to charge the link capacitors. If the voltage is rising above that which is available from the supply, then the required energy must be coming from the load. If this didn't previously occur, then you need to work out what has changed about the overall system to cause an increase in regenerated energy. List the possible explanations, and work through them. Some will be load-related, some drive related. Keep an open mind about the possible causes, until you can prove that they are impossible or at least not plausible.

 

[waross]

How many cylinders in the pump?
The fewer cylinders, the worse will be the effect of a faulty discharge valve.
A bad discharge valve will give the motor a forward kick each revolution.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Electricrifying]

issue not yet resolved, the chocking in slurry line(discharge of the pd pump) resulted in repeated trippings due to dc overvoltage, but the pump is designed for max discharge pressure of 167 bar and the dc overvoltage occuring at any discharge pressure, the pump has 3 cylinders,I have changed the capacitor bank still problem persists

 

[waross]

Have you check the discharge valves for any condition that may allow any back-flow?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Electricrifying]

it is a positive displacement(poston diaphragm pump) pump with pulsation dampners, so back pressure is probably being controlled, isn't it??

 

[waross]

back pressure is probably being controlled, isn't it??

That's the point.
Yes, when the pump is new.
I understand that these are no longer new pumps.
If the back pressure is no longer being controlled as it should be, it may be causing your issues.
The pulsation dampers may be making the issue with a leaking discharge valve worse.
This may not be the source of the issue, but as a possible cause it may be prudent to investigate.
You won't be able to see the kick-backs with an Ammeter but if you are able to "scope the motor current you may see a current dip each pump stroke. Possibly only on one or two cylinders.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Electricrifying]

Actually I have checked the trends of dc voltage vs current but when the discharge line is ok then everything remains constant but at times when their is minor fluctuations of rpm of motor say 30 rpm the current dips and dc voltage shoots up, I just want to know the reason of this rpm fluctuations.

 

[waross]

How many time do I have to suggest checking the discharge valves?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

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