DC essential service system earth fault 6

[Ryc92]

Hi there,

We have a 250V DC UPS system here (CCGT power station), fed by two strings of batteries (125V DC string, 250V across both) and three battery chargers (two dedicated to each string, one with a changeover switch to charge either string).

On the charger for one of the strings (and also the other charger when switched to this string) is alarming for an earth fault on the DC side. This is an ungrounded system, with earth fault protection in the form of an insulation resistance monitoring device (set between 5 and 200Kohm). With the protection relay wound up to max (200Kohm) the alarm still stands.

Unfortunately, we cannot simply turn off this system for obvious reasons, and I am looking for a method for locating this fault on load (if possible). When voltage testing to earth, we also get funny voltages (such as 53V to earth on one side, say minus 160V on the negative side). We still get the 125V across the string, and the polarity is correct (we still get 250V across both). We cannot explain why we get these strange floating voltages to earth.

Any help would be greatly appreciated!

Many thanks, Ryan

 

[Overvoltage]

You're getting the strange voltages because the fault has some resistance and is not directly "bolted". What I have done in the past is to measure the voltages while having someone switch off DC sub-breakers one at a time. Eventually, you should (unless the fault is on the main bus), see the voltages return to normal when one of the breakers is open. Then, start tracing the fault down from that point until you find the problem.

 

[Skogsgurra]

Setting the monitor high will not make the fault go away. Try setting at as low a value as possible to see if it goes away then.

If it doesn't go away when set to 5 kohms, it will probably be possible to drive enough current through the fault to get a traceable current. I have a quite sensisive DC current clamp that I use for similar purposes. It is called I-prober 520 and is very versatile in that it can "sniff" weak DC in PCB traces as well as current in heavier conductors where the 4-20 mA clamps can't be used.

The 53 V indicate that you will get a detectable current if you connect an incandescent lämp from the negative side (where you have 160 V) to ground. You could use a time relay to pulse the lamp on/off at a 1 second rate. That makes tracing a lot easier. Good luck!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Ryc92]

Hi, Many thanks for the replies.

We have just turned the protection down to 5Kohm and the fault hasn't dissapeared on the charger it was on (surely indicating that the IR of the earth path is less than 5Kohm?). Interestingly however, whereas before the charger for the other string of batteries (the two strings are connected in series) did not have the alarm up, it now has after we had turned the protection 'up' from 5Kohm to 200 Kohm. One charger states that the fault is on the negative side, and the other the positive (which is the common point between one string and another to each respective charger).

We have turned off a lot of the equipment but not all; originally it was thought that the fault was only on one string of batteries, although logic dictates that an earth fault somewhere in the circuit would give a low IR reading to whichever charger that was on? (which it does, however one fault is lower than 5K ohm whereas one is lower than around 100Kohm).

 

[raithrovers1]

Is the insulation resistance monitoring device made by Bender? If so this unit causes some very unusual readings when you go between ground and the positive or negative bus. This is due to the way that it monitors faults. Try and disconnect the monitor from the system and then read your voltages.
Is this a new installation? Putting systems in series like this can cause many problems. Is the charger alarming or is it the insulation monitor?
Most chargers with earth fault monitoring do this by connecting some sort of resistor voltage devider between the positive/negative bus and earth(ground). This causes the charger to float at exactly the half way voltage (65V). If you have connected two chargers in series then this can cause issues as the earth will then be tied between the first battery midpoint and then also the second battery midpoint. I suspect this is casuing your problem.

UPS Service Manager

http://www.nolanpower.com

 

[ScottyUK]

Megger make a reasonably good earth fault locator, the BGFT. Bender have something similar, as do Seba. Google might be worth a look.

The old Biccotest 273 Grouser was excellent but doesn't seem to be in production any more. The very low test frequency made it less susceptible than the Megger unit to capacitive losses.

 

[waross]

Rather than having two earth fault systems fighting each other, deactivate the existing systems and instal a system designed to detect earth faults in 125/250 Volt systems.
For trouble shooting, can you run the problem bank on the standby charger and temporarily disconnect the charger that is alarming? If you have a faulty charger, you may be wasting a lot of time looking for a fault in the field.

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

 

[rcw retired EE]

We successfully located 250VDC/125VDC system ground faults using a pulsing AC current injected between the faulted bus and ground. We used a variable voltage ac power supply (0-120VAC Variac) feeding a step down transformer (480v-240v). The transformer output was connected through a fuse and current limiting resistors to earth and the 125/250VDC panel bus. A pulsing relay cut out one of the current limiting resistors to give a varying current easier to detect with the clamp on ammeter. We only pushed enough current to be able to see it on the ammeter, about 1/2-1 amp. With modern meters, you may be able to stay with milliamps.

Clamping around the wires in each of the panels and distribution terminal strips and following the pulsing current led us directly to the faults.

This was in a 1950's vintage plant with few electronics devices on the 125/250VDC system, but a lot of decaying cables and coils that were getting grounds. I was worried about inductive effects or superimposing AC on DC causing issues, but we did not notice any. This method may be a risky with all the electronics on modern DC systems. But it did work well on a GE 7FA CTG control system with no known side effects.

I would also suspect interaction between the two charger circuits as mentioned by others.

 

[raithrovers1]

Scotty
The Grouser is one of the best earth fault testers I have used. Old school in the leather case!I can't find them in the USA but I think they are still available in the UK. I would love to get my hands on one in the USA as I have had many instances where it would come in handy!

UPS Service Manager

http://www.nolanpower.com

 

[Jebb]

I've had some good luck with the Megger BGFT. It pretty much works on the same principal rcwilson describes. I've used the BGFT on systems with lots of electronic devices on it with no ill effects... Good Luck!!!

 

[ScottyUK]

Yeah, the Grouser is an excellent instrument. I think it was an old CEGB design which was put into manufacture - I hesitate to use the word 'production' - by one of their former engineers working from a shed or garage somewhere near Preston. There are copies available from India which are probably made using much the same techniques - internally it was a hotch-potch of stripboard and old-school tape and film circuit board layout.

Regardless of manufacturing techniques it is a very effective instrument for locating earth faults. I had one on hire a year or two ago and it was more capable than the Megger unit unless you get involved in the complex capacitance compensation mode of the Megger unit. I tried it and it is cumbersome.