Mark VIe: Excessive Volt/Hertz Trip

We are in the process of installing MarkVIe to our one turbine. The Exciter control system is also being upgraded to EX2100e. We have installed the new panels and are able to run the turbine till FSNL. As soon as the synch is initiated the lockout relay actuates and turbine trips (L86TGT causes the trip). We also get the excessive volt/hertz trip alarm.

We have the trends on the HMI. The speed of turbine is not changing. Volts do increase. And MVAR value increases to 22 MVAR within 5 seconds and after that the turbine trips. There are no other process and diagnostic alarms.

As we have just commissioned the MarkVIe, the exciter engineer is insisting that the BUS PT and GEN PT connections are not correct although we have connected the signals as per drawings. We are getting two wires for Gen PT volts and two wires for BUS PT volts. When turbine is running on FSNL we get 60Vac on all of the four wires with respect to the ground.

We have turned the Synch Monitor on while on FSNL and connected the analog meter on one wire of BUS and one wire of GEN PT and the voltage goes from 120 Vac to 0. As soon as the voltages reach to 0 the 25A relay turns on for a moment. The same is happening with other two wires of GEN and BUS PT.

Exciter engineer is insisting to check the phase rotation of the BUS and GEN PT. We are getting only two phases for synchronization in MarkVIe panel. Is there any other method to check the phases of GEN PT and BUS PT coming to MarkVIe for synchronization? It would be helpful if anyone is able to explain the relation between MVAR and Generator terminal Voltages. Will Generator terminal voltage increase with increase in MVAR? What can be the possible reason for increase in MVAR and Generator terminal voltage in this situation?
WOW! Just simply, WOW!!!

If the Mark VIe is synchronizing the unit and closing the generator breaker, then the phasing of the BUS & GEN PTs to the Mark VIe IS CORRECT! It may not be correct to the EX2100e, but it's correct to the Mark VIe.

If the generator voltage is rising up very quickly immediately after synchronization either the On-Line Voltage setpoint is not correct, or something is wrong with the On-Line Voltage adjustor or the EX2100e is not seeing the proper generator voltage signal. There should be both BUS & GEN PT wires (signals) going to the EX2100e; they should be verified--as should the field current signal and the generator breaker status signals to the EX2100e.

PT phasing checks can be done with a relay test set, or the old-fashioned way with incandescent light bulbs, or with a 6-volt battery and an ammeter. But, I think the EX2100e has some pretty sophisticated sending and trending capabilities that could probably suffice (if someone knew how to use them).

Excessive volts/hertz is likely being detected because the generator terminal voltage is going too high for the frequency (it doesn't have to be caused by speed (frequency)).

The Mark VIe might be issuing a continuous generator voltage RAISE signal, but it's not likely (unless VAr or PF control is on immediately after synchronization). It's worth eliminating.

But the Mark VIe probably isn't the cause of excessive generator terminal voltage--the EX2100e is the more likely culprit. Based on the information provided.

Nor is PT phasing--at least not to the Mark VIe--because there would be very serious issues with synchronization if this were the case. VERY serious issues.

Please write back to let us know what is found!!!

But, still: WOW!!!

I didn't really address the question of MVAr and generator terminal voltage.

When synchronized to a grid with other generators and their prime movers, when the generator excitation is such that the generator terminal voltage is exactly equal to the grid voltage--the MVAr output of (or input to) the generator will be zero. The Power Factor will be unity (1.0), and there will be NO reactive current flowing in the generator stator windings.

When the excitation is increased from this condition, the generator terminal voltage is trying to "boost" the grid voltage (to increase the grid voltage, so to speak). When this happens (when the excitation is increased above the level that makes the generator terminal voltage exactly equal to the grid voltage) then lagging MVArs flow in the generator stator windings. The generator is said to be "over-excited" and is trying to "produce" MVArs and send them out on to the grid--at the same time that it's trying to increase the grid voltage. The power factor decreases from 1.0 in the lagging direction.

When the excitation is decreased from the condition when the generator terminal voltage is exactly equal to the grid voltage then what happens is that leading MVArs beging to flow in the generator stator windings, and the generator tries to reduce (or "buck") the grid voltage. In this case, the generator is said to be "under-excited" and reactive current flows "into" the generator stator windings from the grid. The power factor decreases from 1.0 in the leading direction.

There are usually buttons on the HMI for the operator to RAISE VOLTAGE or LOWER VOLTAGE--these usually send signals directly to the EX2100e to cause the excitation to be increased or decreased.

During synchronization, the Mark VIe (when it is performing an automatic synchronization) will adjust the generator terminal voltage--usually to make it just slightly higher than the voltage of the grid the unit is being synchronized to. In this way, when the generator breaker is closed there will be "positive" MVArs flowing at the instant of synchronization/generator breaker closure.

If I recall correctly, the EX2100e also has a function that causes the excitation to also be increased slightly immediately after the generator breaker closes--for just the same reason: to ensure positive MVArs flow when the generator breaker closes.

The Mark VIe may have two functions called VAr Control and Power Factor Control. When either of these two functions are active, they will automatically send RAISE VOLTAGE and LOWER VOLTAGE signals to the EX2100e to maintain a MVAr setpoint, or a Power Factor setpoint, as long as either function is active. It's not typical for either of these functions to be active immediately at the time of synchronization, but it can be made to happen.

So, MVArs are related to generator terminal voltage--which is a function of excitation which is controlled by the EX2100e. NO MVARs will flow in the generator stator windings if the excitation level is such that the generator terminal voltage is exactly equal to the voltage of the grid the generator is synchronized to. If the excitation is such that it's trying to raise the generator terminal voltage, and hence the grid voltage, above the point at which they are equal then "positive" MVArs will flow in the generator stator windings. Conversely, if excitation is decreased from the point at which the generator terminal voltage is exactly equal to the grid voltage then "negative" MVArs will flow in the generator stator windings.

MVArs and generator terminal voltage are a function of excitation, which is controlled by the EX2100e. Most often, the EX2100e gets RAISE VOLTAGE or LOWER VOLTAGE signals from the Mark VIe to change excitation, and MVArs. The EX2100e usually has PT (and CT) inputs to calculate MVArs and Power Factor, and it often gets a signal from the generator breaker to tell it (the EX2100e) if the generator breaker is open or closed. These signals ALL have to be correct (phasing; status) for the EX2100e to work correctly.

So, while the sudden increase in generator terminal voltage--and MVArs--is probably NOT caused by the Mark VIe's outputs to the EX2100e, or by the phasing of the PT inputs to the Mark VIe, it's a good idea to check to make sure the Mark VIe isn't sending continuous RAISE VOLTAGE (or LOWER VOLTAGE) signals to the EX2100e immediately after the generator breaker is closed.

Please write back to let us know how you resolve the problem(s)!!!

We were afraid that MarkVIe is sending continuous command to the EX2100e after the breaker closure. We looked into that but it was not the case. Sometimes this happens when buttons on the new HMI are not configured properly. Buttons for EX2100e raise lower command were okay but before that we found speed lower button, which was continuously operating and was not letting the turbine to go above 95%. The CLAMP block was used and its lower limit was 95%. So the turbine was not moving beyond 95%.

The exciter engineer was really frustrated and was trying to shifting the blame on somebody. I do not know whether he was not able to understand the problem or was really trying to shift the blame. He was having more than two decades of experience in Exciter commissioning. CSA, if you can pinpoint the problems even though you are a control engineer (i guess), then I do not know why he was not able to understand the problem even when he was having the support of OEM.

After the three days they used some three phase injection setup and were able to diagnose that a new card in EX2100e was faulty. I do not know the exact details but they have replaced the card and we started the turbine today and it was synched successfully today.

Oh, I know all too well: EVERYTHING THAT GOES WRONG IS THE MARK*'S FAULT. (Be it Mark IV, Mark V, Mark VI or Mark VIe.)

And, yes, exciter field service people also blame most excited problems on the Mark*. Even the ones with decades of experience, unfortunately.

Glad to hear that--once again--the Mark* was not at fault.

VERY sorry (but not surprised) to hear you are having so many problems with the Mark VIe upgradation. But, you got all the latest and greatest safety and performance enhancements that ONLY GE could provide. And, you received as an added bonus problems and issues ONLY GE could have provided, too. Because their upgradation process is just plain wrong and prone to introducing these kinds of problems. The process is also ISO 9000 and ISO 9001 certified!