Earth Fault Tripping During Pump Stopping

Is the earth fault coincident only with the generator supplying the power, or is it persistent after the mains power came back on? Does is happen with only one pump or with all twelve? Do the pumps start and stop simultaneously or are they sequenced in any way? Please advise.

Also, please respond with make and model of soft starter. Some use only 2 sets of SCRs (thyristors), while most use 3 sets. The versions using only 2 sets can cause unbalanced current flow and severe harmonics, which may be falsely tripping your Earth Fault Protection Relay. Itmay be that your acceleration is short enough that it doesn't cause the false trip, but decel ramping in pumps is typically set longer than accel ramping.

Thanks PWSlack and JRaef for being so kind to help, The Earth Fault Tripping only happen when we are using generator's power, when the Utility power resume, it does not happen, but when we switch it to using generator power again, it happen again. The starting and stopping of pumps are controlled by PLC based on the level in the pump sump, no 2 pump will start or stop at the same time, they are controlled to start or stop in sequence, The Brand of Soft starter is Telemechanique by Schneider Electric. Model Altistart 48 .

The tripping happen everytime the pump stop when it is on Generator power but it does not happen when it is on Utility power. Generator voltage and AVR setting are in order.

Hi William Tan,

Your problem is only EFR's input.

This means your generator's Neutral line is not connected to controlled circuit; or you don't earth Generator to your system;

Good luck,

Welcome, William. You've given a good description of the problem. JRaef has a good point about the harmonics. Do the soft-starters provide deceleration ramping on shutdown, or simply shut off all voltage?

Also:

1. Does the generator produce the same voltage as the MCC, or is there a transformer between? In either case, how is the neutral point earthed (direct or through a resistor or reactor)? If the earthing of the generator source is not the same as the utility line, the setting of the EFR may be correct for operation with the utility, but incorrect for operation with the generator.
2. Where does the EFR sense? Is it integral to the starter? In looking at the data sheet, the only protection I saw was overcurrent and phase failure. Is the EFR a true earth fault relay with a CT or shunt in the motor star connection, or does it mathematically calculate imbalance by looking at the 3 phase currents? If the EFR calculates imbalance, and there are capacitors between the disconnect and the soft-starter, the discharge at motor shutdown may be causing your problem. The utility line would have enough single phase load to easily absorb the imbalance, where your small, isolated system with the generator may not be able to do the same.

Beyond the items above, you need to look at everything which is different between the utility and the generator setup. For example, is there a neutral ground connection which gets isolated when the utiity supply breaker is open? Or, is there a thyristor or other field discharge device used in one circumstance and not the other?

Good luck!

Thanks pwr2thepeople, for the nice answer.

As you said, the fault may not be aroused from earth fault. So, first make it sure. Whenever the Generator comes online, the Voltage might be different which may make sense the starters to trip.

What is the nature of both the utility and the genset three phase systems? Do they both have star points that are grounded? If so, are those star points bolted together?

Hi everyone, I've check the voltage and frequency for Utility power and Generator , they are the same about 430v and 50Hz, the AVR of generator is functioning well also, the voltage maintain when I start more pumps.

The Nuetral point of the generator are earthed directly without any resistor or reactor, but I've link all earthing points in the plant to acheive equi-potential.

The type of EFR I use is sensing the inbalance in three phases using three cT, one for each phase and the cT is installed at the cables connecting the outgoing terminals of Soft Starter.

2 x 20Kvar capacitors (525v) are installed in the MCC for each starter and are connected before the soft starter.

What you said is "2 x 20Kvar capacitors (525v) are installed in the MCC for each starter and are connected before the soft starter.".

Since you connected it before the starter, I doubt the stored charge cause to trip the relay. If you have connected them across the motor terminals, it would have circulated through motors windings and will be safe. I am not sure, what I said is correct, still you may think this way also.

Hello William, Go to Set Menu of The ATS48 and access STY parameter...keep it on "F" (which is freewheel stop). Start the pump on generators and check whether fault is reappearing. Good luck,

By the proceedings of this thread i understand that while starting the pumps no problem and while running no problem and when you are required to stop the pump intentionaly the pump motor earth fault relay operates ( The condition is same for all the pumps). That means pump is not tripping on earth fault but the flag is falling when you are stopping the pump. I hope i have understood correctly. This indicates the source is not individual motors and it may the incomer breaker relay from generator side to MCC is at fault. Kindly check the Incomer breaker and revert back

you are right, it only happen when I'm running the pumps using generator's power, I will check the breakers as you said but that I need some time to arrange for a shut down , but to change the capacitor connection may be done immediately, I'll try that, and I'll try to change the setting of the STY parameter also, I'll report the results to you all when I finish the testing , really appreciates your helps. Thousand Thanks.

Dear members, wishing all of you a very Happy New year and excellent health.

I've tested all the suggestions and found that the solution is to change the STY setting to F ( Freee wheel ) plus turn off the capacitors, I have changed the connections of the capacitors to "across the pump terminals " , but the changes in STY to free wheel resulted a big bang on the check valve, I have a big surge vessel in the pumping line but it does not energise due to the water level in the vessel are low, but the big bang in the check valve is beyone the acceptable level. any suggestions?

OK, Guest.

Things are getting more and more complicated. Instead of a pump system that is behaving properly, you are developing a Rube Goldberg (an actual term named after a famous cartoonist who used to portray outlandish systems for doing simple things). In keeping with that image, you might install a constant priming feed through a one-way Vv into the piping before the check Vv, which you would exchange for a remote-control Vv opened by the pump's starting up. This would eliminate the bang by keeping the pipe full. Wheee-ooooooo!!

Or, you could get ahold of the manufacturer and bring a representative around to fix the dang thing for you so that it works the way it was intended to; or if it cannot be fixed, have it replaced for free with one that does.

Also, since you seem to have a head on your shoulders, I'd invite you to join this discussion group as a member so that you could contribute to it from time to time under your own handle.

All the best,

Mark

Ouch ouch ouch!

Do NOT leave the capacitors connected across the pump terminals! They will cause the SCRs in the soft starter to short out! Please people, do not make suggestions like that if you do not know what you are talking about!

Also, you do seem to have another problem, that being that in your hydraulic design you are apparently relying solely on the Soft Stop capabilities of the Soft Starter to prevent water hammer. While I know that some salesman must have sold someone on this concept, realize that he was disingenuous on the concept allowing a total removal of mechanical devices to stop the water hammer. The Soft Stop function must only be used IN CONJUNCTION with a mechanical solution, such as a hydraulically dampened check valve. Here is why; what happens when the power fails? The Soft Starter cannot provide Soft Stopping if there is no electricity! You need to solve that problem regardless of your Earth Fault tripping issue.

Now back to that. When a soft starter goes into Soft Stop mode, it is attempting to reduce motor torque slowly so as to allow the kinetic energy in the moving liquid to dissipate in the system rather than become a shock wave (water hammer). But the kinetic energy is essentially "fighting back" against this reduction in torque from the motor. This causes minor torque pulsations if the decel time is too slow. Schneider uses a "Torque Control" algorithm to accomplish Deceleration at a smooth rate, but this presents a potential problem. These torque pulsations may be the reason for the Earth Fault tripping if the soft starter does not react fast enough in it's control algorithm to prevent them from becoming unbalanced current pulsations, especially if the source power is weak, as in a standby generator. You may not be able to perceive it as anything observable without a scope on the output of the Soft Starter (and even then only to a trained eye), but it may be enough to cause the tripping of a Residual Current based Earth Fault Relay, as you appear to have.

This is not really a big deal, other than the nuisance tripping. If you can, I would see if it is possible to just bypass the EFR sensing by shorting out the CTs with a small contactor only during Decel of the Soft Starters, and maybe even only when running on the generator. The logic would be simple.