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Topic Title: Repeated Alternator Failure on Power Plant rated 16MVA/ 11,000V using 12x 415V generators
Topic Summary: Repeated alternator failure on 12x generators running in parallel
Created On: 06 March 2014 06:25 PM
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 06 March 2014 06:25 PM
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prakashv

Posts: 62
Joined: 19 October 2002

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
 09 March 2014 09:01 AM
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AdrianWint

Posts: 262
Joined: 25 May 2006

I would look deeper into how this system shares the reactive (VAr) power.

The fact that you have several AVR & exciter failures would make me suspicious of the reactive power control scheme and as to whether one alternator is attempting to satisfy the reactive power requirement of the whole system and hence being heavily over excited for long periods of time.
 10 March 2014 08:29 AM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

In addition to Adrian's line of enquiry, another line is neutral earthing. Can you confirm that the LV/11kV transformers are delta on the LV side? How are the generator neutrals earthed?
 10 March 2014 08:48 AM
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prakashv

Posts: 62
Joined: 19 October 2002

There is no chance for one generator to supply the reactive power, as:
1) The DEEPSEA 7510 generator controller is sharing the reactive power communicating with AVR
2) AVR has over excitation protection
3) The rotating rectifiers and rotor winding leads are intact (No signs of any overheating)
 10 March 2014 08:52 AM
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prakashv

Posts: 62
Joined: 19 October 2002

In addition to Adrian's line of enquiry, another line is neutral earthing. Can you confirm that the LV/11kV transformers are delta on the LV side? How are the generator neutrals earthed?

1) The generator neutral are solidly grounded
2) Transformer primary (415V) is delta

Regards
 10 March 2014 05:04 PM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

Originally posted by: prakashv

1) The generator neutral are solidly grounded

2) Transformer primary (415V) is delta



Regards


If all generator neutrals are solidly grounded then you have a path to circulate third-harmonic currents between the generators in the same group. You will get a circulating third-harmonic current if you have differential third-harmonic voltages produced by different generators.

I suggest you try to measure neutral current flowing in each generator under different loading scenarios - third-harmonic current will appear as neutral current.
 10 March 2014 06:07 PM
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statter

Posts: 126
Joined: 06 February 2013

Looks like all the generators that have failed were in the first phase of installation (assuming 2 phases of 6 gens each). Could be a manufacturing defect with the batch. Are there any commissioning test results for the two phases which document insulation resistance and AVR settings?
 10 March 2014 06:45 PM
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prakashv

Posts: 62
Joined: 19 October 2002

Originally posted by: JohnRRussell

Originally posted by: prakashv



1) The generator neutral are solidly grounded



2) Transformer primary (415V) is delta







Regards




If all generator neutrals are solidly grounded then you have a path to circulate third-harmonic currents between the generators in the same group. You will get a circulating third-harmonic current if you have differential third-harmonic voltages produced by different generators.



I suggest you try to measure neutral current flowing in each generator under different loading scenarios - third-harmonic current will appear as neutral current.


The neutral current is less than within limits and generators are 2/3 pitch
 10 March 2014 06:53 PM
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prakashv

Posts: 62
Joined: 19 October 2002

Originally posted by: statter

Looks like all the generators that have failed were in the first phase of installation (assuming 2 phases of 6 gens each). Could be a manufacturing defect with the batch. Are there any commissioning test results for the two phases which document insulation resistance and AVR settings?


The failure is on phase II (6x 1,250kVA generators) installed in 2012. There is no such failure on phase I (6x1,500kVA generators from a different brand) installed in 2010
All 12 generators were tested under load at factory and site for single and parallel operation and ensured everything fine.

I suspect something related manufacturing problem (quality of winding or insulation)
 16 March 2014 04:36 PM
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chrisjtaylor

Posts: 34
Joined: 10 August 2005

Prakashv

Do you have alternator details?

did 2 & 6 fail at exactly the same time? did they fail whilst under load or during switching etc?

Chris

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