Reverse Power Trip & Low insulation

hello guys.
We have serious problems with the online entry of the D / G No3 breaker, it enters manually or automatically for a few seconds, disconnecting urgently. The main breaker was disassembled, checking contacts and mechanisms, it was mounted and the failure continued, so it was decided to exchange the breaker from one generator to another without any result, thus ruling out that the problem is with the main breaker.
The revision of all the circuits and protections of the system is continued, the diode bank of the exciter is measured, the current of the reactor, the card is extracted from the ESM-1102 relay module, and it is checked, cleaned and reassembled. Current and voltage transformers are reviewed.
We shut down the ship and this alternator went online, which does not hold the load at all. The generator was online with the lights, and when we try to start a pump the diesel generator went offline and shutdown.
In the first reactive load that is given to the Diesel generator, it goes offline immediately, so the problem is not that it does not go online but that it does not hold the load after entering. That is why we think that the problem is with the alternator. When we try to give some load to D/G number 3 with auto shift on main Switch Board, the process start and once the D/G 3 receive a load the breaker goes out and then D/G 3 go out of line. We discard the breaker because the ACB of D/G 3 was change to D/G 2 and it’s working property.
We extract the ACB and we can see in the bakelite evidence of electric arc, not normal. We disconnect the cables on Generator (R S T stator coil) and use an insulation tester. The insulation tester result on Terminals R S T of the generator was about 0.5 Mohms. In my opinion it is too low.
We also see that in ACB there is an error RPT(Reverse Power Trip) led is on(red). I've been reading your post about reverse power and we have to check the fuel injection and the governor.
We check some relays that are part of the control circuit of synchronization circuit but nothing happens. we discarded the synchronizer because the other generators go online without problems.
i hope receive your kind opinions.
best regards
Abel
 
hello guys.
We have serious problems with the online entry of the D / G No3 breaker, it enters manually or automatically for a few seconds, disconnecting urgently. The main breaker was disassembled, checking contacts and mechanisms, it was mounted and the failure continued, so it was decided to exchange the breaker from one generator to another without any result, thus ruling out that the problem is with the main breaker.
The revision of all the circuits and protections of the system is continued, the diode bank of the exciter is measured, the current of the reactor, the card is extracted from the ESM-1102 relay module, and it is checked, cleaned and reassembled. Current and voltage transformers are reviewed.
We shut down the ship and this alternator went online, which does not hold the load at all. The generator was online with the lights, and when we try to start a pump the diesel generator went offline and shutdown.
In the first reactive load that is given to the Diesel generator, it goes offline immediately, so the problem is not that it does not go online but that it does not hold the load after entering. That is why we think that the problem is with the alternator. When we try to give some load to D/G number 3 with auto shift on main Switch Board, the process start and once the D/G 3 receive a load the breaker goes out and then D/G 3 go out of line. We discard the breaker because the ACB of D/G 3 was change to D/G 2 and it’s working property.
We extract the ACB and we can see in the bakelite evidence of electric arc, not normal. We disconnect the cables on Generator (R S T stator coil) and use an insulation tester. The insulation tester result on Terminals R S T of the generator was about 0.5 Mohms. In my opinion it is too low.
We also see that in ACB there is an error RPT(Reverse Power Trip) led is on(red). I've been reading your post about reverse power and we have to check the fuel injection and the governor.
We check some relays that are part of the control circuit of synchronization circuit but nothing happens. we discarded the synchronizer because the other generators go online without problems.
i hope receive your kind opinions.
best regards
Abel
Hello..

Would you tell us OEM of designated AVR.. DIESEL GOVERNOR.. GCB.. (ACB)..

Any alarms or trip annuciated on any IED device (AVR. GOVERNOR. PROTECTION RELAY..) like over/underfrequency..
Over/undervoltage



That's right You may have to check if. Diesel engine& (governor).. Are working properly...

Also can you tell us how you proceed when synchronizing those DGs...

A SLD Single line diagram would be a good start.... So it is up to you to share it or not here...

Then we can have a better overview on site situation..

Any time.!

James.
 
ControlGuy25
thanks for reply.
the only alarm is re reverse trip in console and ACB.
The proces of synchronization: We start the Diesel generator and wait for a few minutes to the generator get ready. The voltage is 440V and the frequency is 60 Hz. then we start the proces of synchronization in auto mode. it can also be perfomed in manual mode. once the Diesel generator 3 is going to receive some load, the ACB is close and open again(this i like 1 second), and activated the RPT alarm on ACB and in the console appears the reverse trip alarm.
We carry out the insulation test with an anlogic one, is like a dynamo.
i've attached drawings of connections.
 

Attachments

@ARV1992
Thanks for these inputs!

I see that it is a "Static exciter " on this generator can you confirm..

If so I would advise you to check proper operation & components of the exciter ( like brushes)

When last maintenance have been performed on the Exciter?

Start to search on this path

I will have a closer look on the Drawing and will come back with some notes

Also can you tell us OEM /Manufacturer of Diesel engine governor Generator Exciter..AVR...

Any time!

ControlsGuy25
 
Okay
So let's say that Reverse power can be due to not enough power transmitted by prime mover during synchronization.. Or malfunction of AVR which cannot build up.. Voltage on designate generator according to load consumption..

That's why I asked if there is other alarms/trip that have benn annuciated during that event.

Let's try to get some trends during next sync...

Like:
Output power of DG..
Generator voltage build up...
Fuel reference.. & actual value/setpoint...

Cheers,
Controls Guy25
 
Looks like you got cross curent compensation scheme/mode configured at site...

In case of Island mode you better check proper operation on AVR CLOSE/OPEN COMMAND as I see on the shared drawing...

Something may be not ok on Droop Mode on one of the 3 DG...
 
Also you stated that with reactive load problem occuring too

Let remind the following fundamentals on DG sets:

http://www.pecplc.com/index.php/art...ors-with-droop-and-cross-current-compensation


Island operation with paralleled generators

For the case of island mode operation with at least two generators connected in parallel to supply the load, the control of the voltage and the reactive power requirements have to be shared between the generators in parallel.

There are two control methods for the generator AVRs to achieve this:


  • Control with droop compensation.
  • Control with cross-current compensation.



2.3.1. Control with droop compensation

In this case the following assumptions have to be satisfied:


  • The generators must be of equal size.
  • The AVRs must have the same droop characteristic and the same setting applied.

In the simplest case, the AVRs can operate in droop compensation mode to obtain equal sharing of the reactive load. The relevant diagram is shown below.








Figure 5. Island mode with two generators in parallel in droop mode.


The two generators in figure 5 share equally the reactive load connected according to the droop characteristic of the AVRs and the setting applied.

Although this control mode is ideal when there is grid connection, in island mode it results in the voltage output being dependent on the reactive power demand. So, as the requirement for reactive power increases, the output voltage from the generators decreases due to the droop compensation.


2.3.2. Control with cross-current compensation

Cross-current compensation or reactive differential is a method that allows two or more paralleled generators to share equally a reactive load, given that the following assumptions are satisfied:


  • There is no grid connection, i.e. the generators operate in island mode.
  • The generators are of equal size.
  • The AVRs have the same droop characteristic, which is set to its maximum setting.


The secondary wiring of the compounding CTs of all the generators to be paralleled have to be interconnected. Below, the wiring configuration for two generators set up for cross-current compensation is included.








Figure 6. Island mode with two generators in parallel with cross-current compensation


According to this method, the same current develops through the compounding CT’s of the generators in parallel, since they are identical, and when the CCC contact closes, it stops flowing through the AVRs, but only flows through the CTs.
The configuration above shows that by the operation of a simple contact the CCC can be enabled or disabled, allowing the flexibility to enable it when in island operation and to disable it before connecting it to the grid. This eliminates the droop effect and allows the paralleled generators to operate in island mode at nominal voltage when the reactive load increases.


The figure below shows the complete configuration with all the techniques explained previously incorporated for maximum functionality. This includes both the droop-compensation and the CCC enable-disable contacts.
In this case, when the generators are connected to the grid, all contacts must be open.
For paralleled generators in island mode, the droop contacts must be open and the CCC contact closed.








Figure 7. Island mode with two generators in parallel with cross-current compensation and droop disable contacts.
 
A generator-set will trip on reverse power when the amount of power (torque) being supplied to the generator is not sufficient to “push” real current to the grid or system to which the gen-set is synchronized OR if there is a gen-set on the grid or system to which the gen-set is synchronized can “over-drive” one or more gen-sets synchronized to the grid/system forcing real current to flow “in” to the gen-sets. If the reverse power trip is only happening to one gen-set it’s not likely the latter possibility.

Do you know what governor mode the affected gen-set is operating in at the time of the trip? Is it Isochronous Speed Control or Droop Speed Control?

Is there some kind of power- and/or frequency management system that is monitoring the “board” and sending signals to the gen-sets to share load”? Is this system working properly?

A false reverse power condition can also occur when the sensor(s) being used to monitor current flow out of or into the gen-set are not getting the proper signals or are not functioning properly resulting in erroneous current flow indications.

These power monitors typically use PT (Potentail Transformer) and CT (Current Transformer) inputs. Problems with the transformers or problems with the wiring connections to the power monitors and other devices. So, unless you are certain there is a true reverse power situation with this gen-set’s output you should be checking the power monitor input(s) to the protective relay which is initiiating the reverse power trip.

You would need a means of confirming generator terminal voltage AND generator current—magnitude and direction. Some newer protective relays record and store current and voltage and this information can be downloaded to a PC for review and analysis.

I have witnessed diesel gen-sets that could not get enough fuel to flow to the engine because of either dirty fuel filters or water in the fuel that causes paper in the filter elements to swell and restrict the flow of fuel through the filter. I have also seen diesel gen-sets suffer from low power output because of extremely dirty diesel air filters or because of emergency air shut-off dampers on the air intake which were not fully open.

If you suspect the generator excitation system what reason do you have? Has some work been done on the excitation system recently? Has it been problematic recently? Is it annunciating alarms which give you cause for suspicion)? If so, what are the alarms?

Often, the generator excitation system shares some of the same PT and/or CT circuits as the protective relay(s).

You seem to have done a good job of eliminating possible causes—but you also seem to be fixated on the generator excitation system, and potentially overlooking other possible causes. You might need someone with a means of capturing high-speed data (PT and CT data, most importantly) to review and analyze to narrow down the possible causes to a particular component or piece of equipment. Again, some protective relays have very sophisticated data capture and storage functions which can be very helpful.

Troubleshooting a difficult problem is often a process of elimination. And you seem to be doing that, but you also seem to be concentrating on the generator excitation system but we don’t know why you have decided to do that.

My recommendation at this point based on the information provided is to verify that a reverse power condition is truly occurring—that the device that is tripping the gen-set on reverse power is getting correct inputs. With that information you can decide how to move forward in a logical manner to arrive at the root cause of the problem.

Some governors can also provide useful information on fuel rack position and other parameters that might also be useful.

Best of luck, and please write back to let us know how you fare in resolving this issue.
 
@CSA thanks for your reply.
we pass through governor because if there is any problem with it, the frequency should drop. in this case it seems is an electrical problem (control signals or excitation circuit, our suspicious). The Reverse power trip is on ACB in MSB. The exciter field is controlled by the AVR with the silicon rectifier. we check this by disconectin A conector from AVR and the voltages output rise, then we stop and reconnect the A cable wire and the voltages oes to 440 agin. We changed the silicon rectifier for a new one. if we have a new AVR we can change but there is no one on board. We will follow your recomendations and keep you updated.
Thanks for your kind help
 
ARV1992,

You didn’t answer the question about a power- and/or frequency management system that might be sending signals to the governors (and possibly the excitation systems) OR the governor mode of the affected gen-set….

ALSO, a lot of acronyms are not widely used in different industries. If you use a TLA (Three-Letter Acronym) in a forum like this it’s recommended to explain the TLA—at least the first time you use it. It can improve the quality and promptness of the responses you receive.
 
ARV1992...
Never mind what should happen ! How about listing as many component variables known for each gen-set such as kW, V, A, PF, Exciter V and A, Engine RPM and fuel-rate, and alarms displayed!
Regards, Phil Corso
 
Yeah, well, most original posters these days are not interested in supplying any actionable data such as operating values or alarm lists. They just want to know what to do to fix their problem—plain and simple.

It would be interesting and informative to know how the generator insulation was tested. Was it done using a megohmmeter and what voltage the measurement was taken at. Also, if there were any surge arrestors on the generator output terminals if they were disconnected during the test.

Also, Phil Corso, what would you expect a good insulation reading would be? Would you like to have a PI (Polarization Index) reading for insulation resistance for a more reliable test/indication?
 
CSA...
I don't believe that poor PI had anything to do with Reverse Power Tripping !

ARV stated that the ONLY alarm indication was Reverse Power. His finding that generator stator cable tests indicated that PI was 0.5 Meg-Ohm, which is 500,000 Ohms. It is insignificant and misleading information, and not at all related to the most probable cause "Reverse Power" !

Regards,Phil Corso
 
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