Impossible Flashover on 5KV System

[trosepe]

The System:
The substation at the facility has a 66KV to 4.16KV, Delta-Wye transformer, 5%Z with a 6 ohm resistor grounding the neutral on the 4.16KV side. The main OCD in the plantis a fused air switch rated 1,200 amp cont. and 1,200 amp interrupting. The neutal resistor has a 400:5 CT in series on the low side connected to a ground relay which operates the 4.16KV fused switch at 400 amps.

The 4.16KV load bus has two 700HP, two 500HP, and one 400HP induction motor that start via RVS autofmrs and have 75 to 150 KVAR capacitors connected to each 4.16KV motor that are connected during the first starting stage and stay connected while running. The 4.16KV load bus also has two 2MVA and one 2.5MVA transformers, all 4.16KV delta to 480/277 wye (solidly grounded).

The transformers all supply MCC's with approx 2MVA of induction motor load. There are 200 KVAR (total) of PF correcting capacitors connected to the larger motors on the MCC's which connect when the motor is operating.


Recent System Events and Changes:

1. In Dec-08 one of the 700HP motors had a phase to ground fault. The main OCD on the 4.16KV bus did not open due to a control mistake on the open circuit of the fused switch: (it is the type the takes about 5 seconds to charge and trip when an open signal is given). The cable insulation on both the high and low side of the resistor had thermal damage. The resistor also showed signs of significant over heating (silver solder joints severly overheated, cracked porcelain etc). THe cable was repaired and the resistor found to have the right cold resistance (6.17 ohms).

2. The plant replaced 1,000HP of conventional motor startes with VFD's (480V) between Dec-08 and Mar-09. (no information about input reactors).

3. The plant installed 1MVAR of 'Electroflow Power Conditioners' on the 480V MCC's between Jun-Dec 08.

The Flashovers

According to plant personnel; with-in 5 seconds of starting one of the 700HP motors, there were sparks shooting out of the top of the starter section where the capacitor is located. After opening the disconnect for that starter, they opened the capacitor enclosure and found the end of one of the capacitor fuses melted and 5 or 6 approx. 1/8" diam burn holes in the 12ga enclosure top which was measured at 7.25" from the top of the capacitor fuse.

They tried starting a 500HP unit and the same thing happened. They tried starting the other 700HP unit; same thing. Note all events happened on starting. With 500 tons of frozen ice cream to keep frozen, the pressure was on.

What I Did (the consultant)

My first thought was the neutral grounding resistor was damaged and contributing or causing this event. We took a utility outage and had the neutral grounding resistor by-passed with 750 kcmil cable jumper. The system duty is approx 22K at 4.16KV. I did not reconnect the ground relay due to the slow opwning fused switch and the 1,200 amp interrupting rating. (The by-pass of the resistor is to keep the plant operating until a new resistor can be obtained. My fear was that a fault could open the already damaged resistor.)

After the resistor was bypassed I connected two power monitors to the 5KV system; RPM and Hioki using 35:1 PT's connected wye-wye and the CT's on the main 5KV cables from the utility.

I took the precaution of openeing all the disconnectes on the 'electroflow power conditioners'since they were a new addition to the plant.

The interiors of the 5KV starters were inspected and cleaned. The cleaning was not really needed as they were exceptionally well maintained.

We checked all the 5KV capacitors and found two of the five with two open fuses. The fuses were replaced and all capacitors were tested for IR at 2.5KV and found to have IR values above 5K meg ohms. The capacitance bridge the testing company brought did not work so the capacitance could not be verified.

As soon as the utility energized the 4.16KV service the analog PF meter went to .9 lead with no load.

We started the plant back up beginning with all the 480 volt loads first. The analog PF meter went as far as .7 lead but as 480 volts came on line settled at .9 lead. The power monitors indicated .9 lag.

We opened the 4.16KV starter front panels on one of the 500HP motors to observe the contactors and the capacitor durring starting. The capacitor is in a cell by itself with the closest enclosure surface 7.25" from the top of the capacitor fuse.

We initiated the start and the transition from the start to run contacts went as it should. Five seconds after the run contactor closed the center of one of the fused began to glow red. The red mooved up to the end of the fuse and an arc jumped from the tip of the fuse (where the cable connected) to the enclosure. The arc lasted for approx three to five seconds.

After the smoke cleared the starter was inspected and two of the capacitor fuses and one of the main starter fuses were found to be open.

We decided to remove all of the 5KV capacitors and connective cable. I tested several sections of the 5 KV rated non-shielded cable (capacitor connection cables) in water and none had insulation failures.

We started one of the 700HP motors with the capacitor removed and it started and operated perfectly (plant operator said it sounded smoother than any time he could remember).

We then started three more of the 4.16KV motors without problems.

After the plant was completely on line the power monitors indicated the following:

V(l-l) 4.24KV
V(l-n) 2.45KV
I 650.4 amps
P 4.536MW
Q 1.315MVAR
PF .9539 lag
THD - < 1%

The analog PF meter on the 4.16KV swbd indicatted .95 lead.
We reversed the potential to the meter and it indicated .95 lag, matching the power monitor.
Questions!!

1. Has anyone ever experienced this type of failure with the capacitors? The switchgear is inside, in an air conditioned room with less than 15% humidity. The gear is very clean. The arc I witnessed look something like a Tesla coil dischrge. The 7.25" arc on a 4.16KV system is something I would not have believed unless I had seen it. This equipment has been operating without event for nearly 25 years. Now all of a sudden they have a pile of open 5KV fuses and blackened capacitors.

2. Why is the PF so high with all the 4.16KV capacitors removed? Where are the vars comming from? There are only 200KVAR of PF correction capacitors connected at the MCC's and the 'electroflow power conditioners are disconnected. The FVD's maybe? Do VFD's have that much front end capacitance?

3.Does the capacitor event have anything to do with the unusually high PF?

4. What about the 1,000KVAR of 'electroflow power conditioner' units? Does anyone have experience with these things. I looked inside one and it just looks like a capacitor bank with series harmonic reactors and contactors to step in the bank. Would you guys out there connect 1,000KVAR of these things when the system is at .95 without?

The plant owners paid a huge amount of money for these things with the promise of lower power bills, reduced harmonics and increased system capacity.

I would appreciate any help I can get from our electrical engineering community. Thanks to all who offer opinions.

 

[sibeen]

trosepe, have you managed to find out what the P.F was before the electroflow was installed.

Also, a diagram may help here.

 

[briand2]

Have you considered that the system may have been suffering from resonance at one or more harmonic frequencies, thus creating voltages sufficiently high to damage capacitor banks, etc?

Brian

 

[trosepe]

To sibeen:
It seems that the system power factor was not measured before the 1MVAR of electroflow units were installed. What is even crazier, the PF meter on the main swbd was indicating .95 lead at the time the units were installed. That analog meter seems to have been connected improperly as when the potential was reversed the meter indicated .95 lag which matches the power monitors. I am having our CAD dept prepare a one line and will post it as soon as it is complete. Thanks.

To briand2:
The first thing I thought of was a severe resonance issue, which may be the case, but there is less than 1% THD on the 5KV bus so where is my harmonic source. I am thinking that when the capacitor fuse began to melt; the resuling arcing between each side of the fuse as it opened was the harmonic source for the resonance. That leaves the question as to why the capacitor fuses open in the first place (five differnt capacitors). The < 1% THD should not have been able to to open these 25 amp fuses. Thanks.

 

[Kiribanda]

trosepe,

Just another idea.

Your ground fault current is limited to 400 Amps at 4.16 kV.When the PF improvement capacitors are on line, when there is a ground fault, will the fault current is STILL limited to 400 Amp or will it be higher than that?

I think there had been a ground fault on 4.16 kV system and that ground fault current was more than the allowable 400 Amp.Due to that some caps may have been destroyed. That is why when you started the motors without caps they started fine and was running without any problem.

I suspect after destruction the remaining capacitors may have caused over excitation for induction motors which will give rise to leading PFs.

Does the above make some sense?

 

[busbar]

 
Ted—I am happy that you reported the flashover and are here to talk about it. One question that may help a little sorting this out…do the PF capacitors and ‘electroflow’ units have internal wye or delta-connected capacitors?

 

[trosepe]

busbar,

Nice to hear from you! The 5KV capacitors(which have been removed)were delta connected. The 'run' capacitors on the larger 480V motors are delta. The capacitors in the 'electroflow' units I do not know. Very good question and I will try to find out right away.

 

[ghostbuster7]

We have hit harmonic resonance at the 23rd,21st,13th,11th etc. harmonic and have drawn over 2 times rated current through the cap banks.The banks lasted less then 2 minutes before meltdown occurred.

The point is:if a resident frequency is present on the electrical system(even with low T.H.D. values), resonance can occur if the cap banks and electrical system short circuit values are at a "resonance optimum" situation(note: there are computer programs that can do the math for you.)