I am not good at VFD but I have seen similar problem many years ago in ABB MV drives.
ABB solved the problem by using a relay to connect the 5kV DC bus through discharge resistors when the drive is on idle/standby mode. The relay disconnects the discharge resistor when the drive starts.
The...
Thank you very much for your views.
I was away for a few days and only able to respond today.
I am weighing the different options to see which is the better for the existing site, bearing in mind that spaces at offshore location is scarce and precious.
I was hoping for more comments from you...
Hello,
I have an offshore installation with an 11kV bus, like this:
- Five 11kV generators connected to the 11kV switchboard, 4nos 2.7MW and 1no 4.2MW, installed at different time.
- All generator neutrals are brought to a switching (contactor) panel so that only one neutral is grounded.
-...
Thank you waross. You are very precise in getting details and I really appreciate it.
Now, answers to your questions:
1: There is a cable from the generator star point to the system neutral bus.
ANS: Yes
2: There is a cable from the generator star point to a disconnect switch, from there to a...
Q: Do you have any single phase loads that are connected from phase to neutral?
Ans: Yes, see Item 4 above.
Q: Do you have one ground measuring ct per machine, or one ct for the whole system?
Ans: One ground measuring CT per machine. The CT is mounted between the isol switch and the earth bar...
No, I am very sure that it is measuring the ground current because the CT is located just before the earth bar and after the T-off from neutral.
The gensets are all rated at 300kVA, rated current 417A.
Waross, thank you for the detailed information and story.
I have more findings on the installation:
1. All generator neutrals are brought to the switchboard and connected to the earth bar there. However, the earth bar is not isolatable because all other components in that switchboard...
I am studying reliability of an old installation and I am asking the client a question on why all four 415V generator sets have their neutrals solidly grounded(3 sets in normal operation).
Can you give me opinions on whether there is a way to mitigate the flow of unbalanced current through the...
Sensitivity of ELCBs should be in mA, not ms.
To check nuisance tripping of ELCB, I always do in the following sequence:
1. Somewhere inside the house may have a little earth leakage that has not reach upto the tripping level. A lightning is just a little surge to add into the leakage. You...
I believe DickDV has misunderstood about DC HOLD and DC braking. These are the texts copied from the ABB manual:
Quote:
2104 DC CURR CTL
Selects whether DC current is used for braking or DC Hold.
0 = NOT SEL
. Disables the DC current operation.
1 = DC HOLD...
Our VFD driven motor has a separate motor driven cooling fan. Therefore, when the motor is under DC current braking condition, it is still being cooled by the fan.
However, the DC holding torque (current) level is adjustable as you wish. Base on our experience, the DC holding current level we...
I had done that successfully with ABB ACS600 series inverter.
I had a motor driving a cryogenic(liquid nitrogen) pump at 4600rpm and we need in inverter to run the motor at 77Hz (This had saved a mechanical gearbox).
The pump is required to operate at -150deg.C and it is only required to be...
msherb, you have a good point that sacrificing the motor is a way, similar to what we had done on our Fire Water pump at higher overload setting.
LionelHutz & jraef's ideas is definitely workable.
I will also looking into setting this alarm from the DCS (or the Safeguarding system), where the...
Thanks to edison, waross & jraef. However, I have a different idea, I don't want to prevent starting of the motor if the timer is not expired yet. The alarm I am asking for will be used to tell maintenance personnel to search and fix the N2 leak.
The HP Nitrogen system is critical for...
