Repeated Alternator Failures on a Power Plant rated for 16MVA /11,000V using 12x 415V Generators.

[vp1959]

I am seeking expertise review to identify the reason(s) for repeated alternator failure on an 11kV power plant rated for 16MVA using 12x 415V generators arranged as per the following, installed/ commissioned in two phases and operating all in parallel with necessary protection and load management system:

1) The first phase commissioned in 2010 is having 3x 3,000kVA power sources and each source consists of 2x 1,500kVA generators supplying 11kV through 1x Synchronizing panel, 1x 3,000kVA, 415V/ 11,000V (D/Y) transformer, 1x neutral grounding resistor (NGR) and 1x Ring Main Unit (RMU)
2) The second phase commissioned in 2012 is having 2x 3,000kVA power sources and each source consists of 3x 1,250kVA generators supplying 11kV through 1x Synchronizing panel, 1x 3,000kVA, 415V/ 11,000V (D/Y) transformer, 1x neutral grounding resistor (NGR) and 1x Ring Main Unit (RMU)
3) All 5x power sources are connected for parallel operation through the RMU with MICOM protection relay for over current, short circuit and ground fault
4) All 12x generators are provided with synchronizing and load sharing controllers (DEEPSEA 7510) for automatic load management (start/ stop based on power demand) with protection for over current (110%), short circuit current (300%) and reverse power (5%).

The first phase (3x 3,000kVA) installed 3years ago and operated over 8,000 hours is working without any problem, but second phase source (2x 3,000kVA) installed two years ago is having repeated alternator failures (different brand from the first source)
- The generators # 2 & 6 subjected heavy flash over in the generator terminal box after 4months from commissioning (Jul/2012) without any apparent reasons. Manufacturer/ dealer is informed, but couldn’t identify the reason and repaired suspecting less clearance between the bus bar though it is same on all sets.
- Generator # 6 is failed again after 10 months of operation (May/2013) due to main rotor (low insulation resistance), exciter stator (brunt), droop transformer (burnt) and AVR (faulty). Manufacturer/ dealer is been informed, but couldn't identify the reason and supplied replacement components after suspecting/ blaming overload, unbalanced load, maintenance, tropical weather etc. though there is no chance for overload, unbalance load and maintenance and weather conditions are common for all sets.
- Generator # 5 is failed after 17 months of operation (Aug/2013) due to low insulation resistance of main rotor and defective AVR. It became okay, as we dried the rotor and replaced the AVR
- The generator # 2 is failed again after 13months of operation (Aug/ 2013) due to main rotor (low insulation). As usual, the manufacturer is suspecting the overload, unbalanced load, maintenance and weather conditions, though it is noticed that there are trace of melting insulation (silicon like material), brittle of rubber tape insulation (common on other sets which are still working) and one of the coil is open circuited as the strip type winding wire for about 20cm is missing (It seems one end is cracked and the other end is tear off)

Regards

Prakash

 

[djs]

Generator rotor failures/over heating are usually caused by excessive KVAR. Excessive KVAR on parallel generators results in excessive rotor excitation which causes the rotor to fail. Another possible cause might be vibration.

 

[catserveng]

Is this an island mode system supplying a load independently or paralleled to a grid or a large network? What kind of load are you supplying?

By some of the failures you are describing it sounds like you maybe are having VAR share issues, but to be sure would require a lot more information from you.

You also mention you "dried the rotor", so are you dealing with extreme environmental conditions? Coastal environment, high humidity, corrosive environments? Could also contribute to your "flash over" event.

Besides being a different "brand" of generator, are the constructed or rated differently? How about winding pitch, are they compatible?

Mike L.

 

[vp1959]

Hi DJS & Catservengg

The system is running on island mode and feeding a labor camp, major loads being air conditioners (split & package type mostly 3ph), kitchen equipments (electric cookers, ovens, mixers), lighting, television, etc.

There was no vibration issue and no chance for high KVAR as the kW, KVA, Amps and KVAr, as well as system voltage and frequency are being monitored and recorded on hourly basis. Moreover, the load management system is set to limit load less than 70% which can easily tolerate the high summer temperature 52deg.C and kW & KVA unbalances if any.

The loaction is desert with tropical weather (regular sand storm, varying humidity and temperature, etc.)

They all identical (rated for 415V, 50Hz, 4Pole, 2/3pitch, etc.), except six are from one brand rated for 1,500kVA (no problem so far) and the other six from another brand rated for 1,250kVA.

Also, please note:
1) The rotors are failed on sets that had subjected the flashover about 10 - 13 months ago
2) If it is KVAr issue, stator should not overheat/ fail? why winding wire broken off?

 

[waross]

Do you have anti-condensation heaters on all sets?

It became okay, as we dried the rotor and replaced the AVR

This is a red flag!! The set shouldn't be getting that damp at any time.

There was no vibration issue and no chance for high KVAR as the kW, KVA, Amps and KVAr, as well as system voltage and frequency are being monitored and recorded on hourly basis.

If the AVR is over or under exciting it will not show on the system meters. The system KVARs are determined by the load characteristics.
An over excited generator will take a greater share of the system reactive current. The system KVARs will remain the same but the KVARs of the other generators will drop.
How well balanced is your load? Unbalanced loading leads to rotor heating and some sets may tolerate overheated rotors better than others.

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

 

[vp1959]

Hi Warross,
The alternators are provided with anti-condensation heaters and they are ON during idle condition. One of the alternators became okay as the cleaning (high pressure air) and drying (heating with flood lights) may have removed the dust or moisture or both bridging the exposed insulation to the ground (I doubt the quality of insulation)
There is no chance for supplying greater reactive current and unbalanced load from one or two generators, as:
1) The generator control is sharing the active and reactive load equally (DEEPSEA 7510) communicating with AVR and governor
2) Such kW and KVAr unbalance never monitored by the operators who check/ log on hourly basis
3) All six generators are same brand, model and AVR are having over and under excitation protection.
4) Phase current also balanced within 10% and maximum continuous load is limited at 70% (Next generator shall be started and paralleled automatically when the load increased above 70%)
Moreover, we never had any problem with other 6x generators, but different brand and capacity (250kVA more) running parallel with subject generators (DEEPSEA 7510 is sharing load based on the capacity)
If environment is suspected, it is common for all 12 generators and hundreds of generators operated in the same areas

 

[vp1959]

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[vp1959]

Please see the attached photos of one of the alternators failed (flash over first and later rotor)

Also we lost another alternator (same brand & model) due to stator failure (stator winding burnt). It was running alone (island mode) in place (Kuwait) where no much humidity and load is below 80%

 

[waross]

It looks as if the anti-condensation heater may have failed or are too small. I have run generators on the shore of a tropic island, about 200 feet from the sea shore. The generators were old surplus units that had seen duty on off-shore oil rigs. None of them had the degree of corrosion on the hardware that you have.
Also, have you considered that rodents or insects may be contributing to flash-overs?
Comments Mike??
Re Kuwait and low humidity. Condensation is a result of thermal lag and rising temperature. When the ambient temperature is rising, the temperature of the rotor and stator lags behind and may be quite a bit cooler than the ambient temperature. If the dew point rises above the internal temperature of the rotor and stator iron, you have instant condensation. Lots of it. Condensation will continue to accumulate until the internal temperature rises above the dew point.

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

 

[catserveng]

Well, the amount of corrosion present in the pictures sure indicates something in the operating environment is corrosive. I have had experience with units coming back from Kuwait where we found the dirt/dust inside the generators was very alkali, and during shipment back from overseas the units returned to southern California with severe corrosion, our theory was as the units were exposed to any moisture at all, it caused the corrosion. I also see this in units operating in desert conditions in places like quarries and cement plants. We also had repetitive failures of generators operating in a waste water treatment plant that was reclaiming water, found that the ambient levels of ammonia and chlorine was attacking the generator insulation. So just because you think you are in a "dry" environment, doesn't mean something can't damage the insulation. And insulation packages vary between manufacturers, could be your one batch of tailends just can't cope with your environment.

Bill's comment on rodents and insects is also a good point. Spider webs by themselves aren't usually conductive, however generate a large number of events because of all the stuff they catch and hold. Rodents not only bridge live equipment, their waste is VERY corrosive. And snakes are one of my biggest revenue producers for generator sets and switchgear, especially mounted outdoors. Your flash over events may also be the result of some kind of transient voltage event, which could also affect the rotors as well if severe enough.

The DeepSea controllers do work well when properly setup, however I recently did a project where everything looked fine on the displays and found the excitation to be pretty unstable on some of the units, causing some repetitive AVR failures. Sometimes dealing with units in parallel the system can mask what is actually going on with a particular generator. Some better tuning of the VAR control dynamics and a change to the stability settings of the AVR's fixed the problem.

And yes, it could just be you have a crappy bunch of tailends, not suited for continuos or severe service. Have you monitored excitation (field amps and volts) to see if in fact the different tailends are working within their operating limits? How about stator temps, how do they compare to the other units with the different generators?

Understand you monitor every hour, but depending on what kind of metering you have and the diligence of our operators, there could still be some kind of interaction going on that may be the cause or a contributor to your problems. Likely you will need to go a few extra steps and do some further electrical analysis, of find a competent expert to come in and review your system.

Mike L.

 

[vp1959]

Hi catserveng

I don’t know where you have seen corrosion in the photos?

I do agree that the reasons specified for corrosions, but there is no such corrosion in the subject generators. However, as you mentioned, I strongly suspect that the failure is due to substandard insulation system, probably on this brand or certain batches, as the environment is common for hundreds of generators operating in the same area from different brands.
Based on my long experience with offshore (barges, ships, etc.) and onshore (deserts, costal, etc.) fields in UAE & Kuwait, I never had such recurring failures on any other generators operated even exposed to rain, fog, humidity, oil, etc.
Many generators are working with their windings fully wet with oil entering from rear seal failure and accumulated dust on it and marine/ shipboard generators are working even worse condition (cooling air with oil mist, salt, soot, etc.)
There is no chance for rodents or insects. However, suspect transient voltage, but no idea due to what it is?
I haven’t experienced any significant inaccuracy issues with DEEPSEA controllers like 5510, 7510 & 8610/ 8660 used for parallel operations. The voltage and current have been checked with other calibrated meters on single and parallel modes during FAT and commissioning to ensure proper loading (0%, 25%, 50%, 75%100 & 110%), voltage & frequency regulation, winding temperature and DEEPSEA readings, but the excitation voltage and current were not checked.
In case the rotor is failed due to over excitation, it shall be due to rotor problems and AVR should have prevented it (by over excitation). The rotating rectifiers and rotor winding leads are intact, but AVR and droop transformer used to fail with the flash over, rotor…..
Manufacturer representative has inspected the generators and we are waiting for their report, but I don’t think they will reveal the real cause as the warranty is over and they are defensive (environment, maintenance, overload, unbalanced phase voltage, etc.) to protect their reputation.
We used to have voltage build up problem on STAMFORD alternators supplied before 2000 when there is fog or high humidity, but it never shorted/ burnt the windings and we used to wait for the fog out or heat the alternators for urgent power demand and upgrade the windings with additional coatings, relocate the AVR inside IP55 enclosure etc. to overcome this problem, but such issue reduced with the later generators, probably they have identified the problem and improved.

Regards

 

[ScottyUK]

I see corrosion on steel bolts and on the casing, but more of interest is the blackened copper. Both are indicative of hydrogen sulphide attack, which is aggressive even in low concentrations. What is the plating on the larger busbars - tin or silver? Look up 'silver whiskers' as a possible initiator for the flashover. If the rotor bars are silver brazed then that's another possible area for concern.

 

[vp1959]

Hi ScottyUK
Such corrosion on exposed/ uncoated steel are common here, including on stainless steel, galvanised steel, etc.
If the blackened area you mensioned is in the rotor, then it is not corrosion, instead it is exposed winding/ varnsih (the area where the winding strip broken/ lost).
The bus bars for conncecting alternator leads are tined while the bus bar for connecting output cables are copper (no coating)
The flash over occured on both, nut majority on tined copper
The rotor winding connection are sealed/ insulated (Unable to know how they made the joints)

Regards

Prakash