GT Start Up monitoring parameters after Reverse Power/ Generator Motoring

One of our unit (Frame 5PA) recently had a reverse power, 30 minutes after starting the unit with the power generating 11MW and power dropping (motoring) to -9.52MW for several minutes.
Generator Breaker is still closed at the time of event and can't be manually opened, therefor causing the operator to shutdown the unit via ESD button but the turbine still rotating and Breaker still closed. Operator then activated 12HA (Overspeed Bolt Trip) but the result is still the same. Operators then closed the incoming gas line through manual valve upstream and downstream of strainers with the same result. Operators then finally open 115kV GCB which finally caused the turbine to slow down to 0 RPM.
Flag/alarm that pops up in the HMI at the time of the event is "Manual Trip", followed by "Overspeed Bolt Trip", "Gas Fuel Pressure Low" and "Generator Differential Trip" which in my opinion is a result from the actions of the operators, but does not show flag is active during the initial reverse power/generator motoring.
Note: There is no data trender/logging on this unit, so I can't check parameters such as vibration, winding temp, RPM.

Relay 32 (Directional Power Relay), 40 (Loss of Excitation) and 64F (Field Ground Protection) were found to be active/flag.

The next day, we decide to visually inspect the generator rotor and stator (winding at load gear side and pedestal side), which there seems to be no anomaly or any significant damage other than slightly dirty.
Visual inspection on Busbar, PT, CT Excitation Trafo, Fuse for PT&CT, Fuse Control all seem to be normal.
Resistance test is conducted on generator rotor with result of 76.9 milli ohm and a phase to ground megger resulting in 3.04 Mega Ohm, which seemed acceptable.
Generator Breaker during inspection seemed to be melted, which caused the it to be closed during the event and repairs have been done and also inspection to other generator protection relay is conducted for assurance

Based on the findings above, it is planned that the unit will be started again (at least until full speed no load) in the near future but I still have some doubts whether it is the best path forward as I don't have the winding temperature data and vibration data during the motoring which might give an insight whether damage will likely to occur or not.

But if the turbine is still decided to be started until FSNL, is there any specific parameters that must be observed which might indicate that there is damage from the generator motoring? or is parameters such as generator & load gear vibration, generator RPM, end winding temperature, excitation current monitoring sufficient ?
Is there any further testing required before I can state the unit can be deemed ready to start?


Appreciate your input
*Apologize if there are any error as English is not my first language.
 
Txture,

A good explanation of the issues, but two things seem to be missing: the type of excitation system, AND whether or not the excitation system was working during the reverse power condition.

If the excitation system was not working during the reverse power condition (for just about any period of time during that condition) induction heating of the generator rotor will occur--and it can happen relatively quickly. So, the problem isn't so much stator heating as much as it is rotor (over-)heating.

Meggaring the generator rotor, can be difficult because it's necessary to be sure the generator rotor is NOT connected to ground through a generator bearing. If the unit has a brushless exciter that can also contribute to issues with checking resistance. It's not clear if you performed a PI (Polarization Index) test on either the generator stator or the generator rotor. In my experience those are really the best tests when initially checking a generator for any kind of damage.

But, I think you have pretty much listed all of the available data which can be monitored during FSNL operation. I would suggest someone look over the generator excitation system for any signs of damage or problems (overheating; blown fuses; etc.). And, if there is any question at all about the condition of the equipment (excitation system; generator stator; generator rotor)--call in a trusted testing company and knowledgeable individual(s) to help protect against unintentional damage.

Hope this helps! Please write back to let us know how you fare with your testing and the outcome(s).
 
Txture

The Reverse-Power relay (ANSI/IEC Tag '32') is generally used to protect the prime mover, especially if a gas or steam-turbine. Damage to the generator is harmful if resultant currents are unbalanced. The melted breaker components (stabs or bus ) are certainly indicative of major current amplitude! But power values listed are not high! Certainly not high enough to make a call related to generator damage ! Operation of the Ground-Fault Relay is also unclear because, presumably, the system or generator neutral is resistance, or impedance grounded !

1) Please provide a Single-Line-Diagram of the principal components and heir ratings, especially those of the generator, connection to the load, or grid, etc.

2) Please provide a list of Protection Devices (ANSI Notation Preferred) installed, including name of supplier or mfg?

3) You produced some parameter values and their duration! Does the protection system include a Sequence of Events Recorder ?

Regards, Phil Corso
 
Nasser56&,

This thread is about GE-design Frame 5PA.

If you want better luck with obtaining a GE-design Frame 9HA Operations Manual you should open a new thread with your request in the Subject line, and in the body of the thread.

Blessed day.
 
Sorry for the late reply, as we were still focused on the GT unit (plan to conduct FSNL tomorrow). To answer some of the data requested above:


Hi,
Can you provide more details such as, turbine control system type, GCPP, GEP.
Hi,
The control system is a SP Mark-IV+, SLD is provided for further info, hopefully it is sufficient.

Txture,

A good explanation of the issues, but two things seem to be missing: the type of excitation system, AND whether or not the excitation system was working during the reverse power condition.

If the excitation system was not working during the reverse power condition (for just about any period of time during that condition) induction heating of the generator rotor will occur--and it can happen relatively quickly. So, the problem isn't so much stator heating as much as it is rotor (over-)heating.

Meggaring the generator rotor, can be difficult because it's necessary to be sure the generator rotor is NOT connected to ground through a generator bearing. If the unit has a brushless exciter that can also contribute to issues with checking resistance. It's not clear if you performed a PI (Polarization Index) test on either the generator stator or the generator rotor. In my experience those are really the best tests when initially checking a generator for any kind of damage.

But, I think you have pretty much listed all of the available data which can be monitored during FSNL operation. I would suggest someone look over the generator excitation system for any signs of damage or problems (overheating; blown fuses; etc.). And, if there is any question at all about the condition of the equipment (excitation system; generator stator; generator rotor)--call in a trusted testing company and knowledgeable individual(s) to help protect against unintentional damage.

Hope this helps! Please write back to let us know how you fare with your testing and the outcome(s).
Hi CSA,
Excitation system is a static type and fortunately was running during reverse power. PI is conducted on both rotor and stator as a follow up and the result is 2.93 for stator and 3.55 for rotor which seems acceptable.

Txture

The Reverse-Power relay (ANSI/IEC Tag '32') is generally used to protect the prime mover, especially if a gas or steam-turbine. Damage to the generator is harmful if resultant currents are unbalanced. The melted breaker components (stabs or bus ) are certainly indicative of major current amplitude! But power values listed are not high! Certainly not high enough to make a call related to generator damage ! Operation of the Ground-Fault Relay is also unclear because, presumably, the system or generator neutral is resistance, or impedance grounded !

1) Please provide a Single-Line-Diagram of the principal components and heir ratings, especially those of the generator, connection to the load, or grid, etc.

2) Please provide a list of Protection Devices (ANSI Notation Preferred) installed, including name of supplier or mfg?

3) You produced some parameter values and their duration! Does the protection system include a Sequence of Events Recorder ?

Regards, Phil Corso
Hi, to answer some of your questions:
1. SLD Attached
2. List of Protection Devices are as follow:
ANSI Code Function
24 Overfluxing
25A Synchronizing
27B Undervoltage
86G Lock Out Relay
87G Generator Differential
60 Voltage/Current Balance Relay
46 Phase Balance Current Relay
40 Loss of excitation
32 Reverse Power Relay
21GB Ground Distance
62 Time-Delay Stopping
64 Ground Protective Relay
64F Field Ground Protection

I currently don't have the name of manufacturer, but i can check the manual but I'll be.

3. Unfortunately a sequence of events recorder only recorded the following "Manual Trip", followed by "Overspeed Bolt Trip", "Gas Fuel Pressure Low" and "Generator Differential Trip"
 

Attachments

Txture...

1) Please send data or pictures reflecting the location(s), and damage severity
to equipment shown on the SLD. For example, on line-side or gen-side of 52G. As second example, Gen-side or Grnd-side of NGT ! Etc ?

2. Please indicate, with an "X" mark which of the Protective Relays listed had operated ?

3) By now do you know why the SER, "unfortunately" failed to work ?

3) What is your level of experience ?

4) Relay MFg names are moot at this time!

Regards, Phil Corso
 
Apologies for the late reply, got the case of C-19 back to back and unfortunately forgot to post update.
Luckily, as the excitation is running during reverse power, it seems that there are no significant damage on the generator and or excitation.
The unit has been successfully operated on safe conditions with no parameters (temperature,vibration,etc.) affected.

Thank you for the inputs and insight.
 
Top