Ship Generators Problem Running In Parallel - KVAR Unequal

Recently been having problems on ship with our 2 Caterpillar 3406c Generators. Both generators are identical and both using EMRI CAT v2.3 AVRs.

When both are running in Parallel, both generators seem to be sharing load (kW) equally, but we are noticing these things:

Generator 1 has a much higher KVAR reading and lower power factor (around 0.5-0.6 ish) compared to the other generator. The main breaker for this generator is also showing much higher current values, sometimes even nearing 100% load on the breakers trip unit.

Generator 2 is reading close to 0 KVAR and sometimes it’s even going to around -3 KVAR, and it also has very close to 1.0 p.f. The current values on the breaker for this generator are much lower compared to the other.

We recently replaced the AVR and Rectifier Diode Bridge on Generator 1 as we had a problem with one of the exciter fuses blowing, which we don’t 100% know the cause for.

Any solutions or explanations as to why this is the case would be appreciated.

Thanks
 
Gregorb6
In all probability, and depending on the ship's age, the cause is related to a common, but simple, paralleling control-function called "Reactive-Current-Compensation", or just "Cross-Current-Compensation". I suggest you use the FORUM's "Search" feature for it as well as "Unequal Generator Load Sharing."
The problem is also relatively easy to trouble-shoot, at least in my day before the digital age.
Regards, Phil Corso
 
Recently been having problems on ship with our 2 Caterpillar 3406c Generators. Both generators are identical and both using EMRI CAT v2.3 AVRs.

When both are running in Parallel, both generators seem to be sharing load (kW) equally, but we are noticing these things:

Generator 1 has a much higher KVAR reading and lower power factor (around 0.5-0.6 ish) compared to the other generator. The main breaker for this generator is also showing much higher current values, sometimes even nearing 100% load on the breakers trip unit.

Generator 2 is reading close to 0 KVAR and sometimes it’s even going to around -3 KVAR, and it also has very close to 1.0 p.f. The current values on the breaker for this generator are much lower compared to the other.

We recently replaced the AVR and Rectifier Diode Bridge on Generator 1 as we had a problem with one of the exciter fuses blowing, which we don’t 100% know the cause for.

Any solutions or explanations as to why this is the case would be appreciated.

Thanks
Hi all,

First as suggested by Phil Corso , you should have some read if not yet on cross current compensation/droop mode compensation for Gensets operating in parallel.

Here is a link for a well explained principle of cross current and droop compensation:
http://www.pecplc.com/index.php/art...ors-with-droop-and-cross-current-compensation.

When you saying that fuses blowing , can you clarify .. is that Permanent Magnet generator called also PMG that you are talking about?

When fuses are blowing it is an indication that something is not good /operating properly.
That can be a burned diode, a short circuit, or something else...

Without be able to check the system drawing and alarms /trip log of controls system, it would be difficult to give more support.

Do you get any alarms /trip annunciated ?

I will have a better read on the original post and see if I add more comments for best troubleshooting.

Hope this can help,
Controls Guy
 
Check this very useful document to have better overview of what the system should do/work in smooth manner:

https://fr.slideshare.net/Otorongosabroso/cdvr-service-manual-renr794100-sis-caterpillar
It may not use the same AVR but philosophy remains the same!

The manual explaining lot of operations/tests method to get your system working properly according to the respective load .

Can you share a SLD ( single line diagram) of your electrical system, and also geenrator wiring ( exciter/field ...)
With these drawing we can have better overview and give better advices.

I am sure that can highlight you some points that you are looking for .

ControlsGuy25.
 
Gregorb6...
The simple trouble-shooting test I alluded to earlier was to short-circuit just the two CT's involved ! If load-sharing returned via manual-control then the cause was the Cross-Current-Compensation, or CCC function!
Regards, Phil Corso
 
Check this very useful document to have better overview of what the system should do/work in smooth manner:

https://fr.slideshare.net/Otorongosabroso/cdvr-service-manual-renr794100-sis-caterpillar
It may not use the same AVR but philosophy remains the same!

The manual explaining lot of operations/tests method to get your system working properly according to the respective load .

Can you share a SLD ( single line diagram) of your electrical system, and also geenrator wiring ( exciter/field ...)
With these drawing we can have better overview and give better advices.

I am sure that can highlight you some points that you are looking for .

ControlsGuy25.
Can the original poster give us some feedback....
 
Gregorb6
In all probability, and depending on the ship's age, the cause is related to a common, but simple, paralleling control-function called "Reactive-Current-Compensation", or just "Cross-Current-Compensation". I suggest you use the FORUM's "Search" feature for it as well as "Unequal Generator Load Sharing."
The problem is also relatively easy to trouble-shoot, at least in my day before the digital age.
Regards, Phil Corso
Hello Gregorb6

Can you please elaborate, that how does the age of ship affects the sharing of KVAR between two generators running in parallel?

Let us say we have two Diesel Generators of same make and rating (1990 model, 1100KVA), both the generators have same AVR (initial settings adjusted to 2% Quadrature droop for both generators), we have observed that during initial days of commissioning the sharing of KVAR was pretty decent, however as the time progressed (approx 25 years) there is a prominent difference in the sharing of the KVAR.

During every operation, manual intervention is required to ensure the currents doesn't shoot up.

Can you please throw some light on the reasons for the same.

Regards
 
Hello Gregorb6

Can you please elaborate, that how does the age of ship affects the sharing of KVAR between two generators running in parallel?

Let us say we have two Diesel Generators of same make and rating (1990 model, 1100KVA), both the generators have same AVR (initial settings adjusted to 2% Quadrature droop for both generators), we have observed that during initial days of commissioning the sharing of KVAR was pretty decent, however as the time progressed (approx 25 years) there is a prominent difference in the sharing of the KVAR.

During every operation, manual intervention is required to ensure the currents doesn't shoot up.

Can you please throw some light on the reasons for the same.

Regards
Hello All,

Is there any kind of Load sharing module installed on this plant..

The reason for the same would be cross current compensation /kvar droop compensation scheme to study ..

Maybe onb eof the 2 AVR is not well tuned and send overexcitation on one genset..causing KVAR fluctuations between both Gensets...

Here some notes to take in account :

REACTIVE POWER (KVAR) LOAD SHARING
  • When generator sets operate in parallel the alternator field excitation system of each generator set controls the proportional sharing of the total reactive power requirements (kVAR) of the system.
  • The kVAR load sharing is achieved by increasing or decreasing the field excitation to the systems’ alternators.
  • As the field excitation of one generator set in a group is increased i.e. overexcited it will not lead to an increase in voltage (as it would if it were operating alone) but it will lead to an increase in the proportion of the total kVAR load it will deliver and a decrease in its power factor.
  • As the field excitation of one generator set in a group is decreased i.e. under-excited it will not lead to a decrease in voltage (as it would if it were operating alone) but it will lead to a decrease in the proportion of the total kVAR it will deliver an increase in its power factor.


An undesirable circulating reactive current (cross current) will flow in the system if the excitation of the alternators is not matched.

  • The voltage control system of the generator sets (via the alternator voltage control system) monitors and controls the sharing of the total kVAR load in proportion to the relative rating of the alternators on the systems’ generator sets.


Any time!

Controlsguy25
 
Cross current compensation
Cross current is a flow of electrical current between
generator sets that is caused by dissimilar excitation
levels in those sets. Cross current compensation is a
www.cumminspower.com
©2010 Cummins Power Generation
term describing the operation of paralleled generator
sets without intentional voltage droop. This is achieved
by the insertion of a current transformer (CT), usually
on “B” phase of each generator, and interconnecting
the CTs together to provide an identical voltage
bias to each AVR in the system. The system works
best when the voltage regulators are all of the same
manufacturer and model. Not all voltage regulators
work together in this mode, so the best planning
practice is to make sure that all the voltage regulators
in a system that uses cross current compensation are
identical. This may require changing all the voltage
regulators in the system to a new model. Using cross
current compensation results in no intentional droop
in voltage from no load to full load on the system,
so it is considered to be superior to a reactive
droop compensation system from a performance
perspective
 
Droop governing and voltage regulation (a.k.a.
“reactive droop compensation”)
Isochronous kW load sharing
Isochronous voltage kVAR load sharing
Cross current compensation for kVAR load sharing
 
Load sharing factors
When a generator set is in a paralleled
arrangement, the voltage and frequency
outputs of the generator sets are forced
to exactly the same values when they are
connected to the same bus. Consequently,
generator set control systems cannot simply
monitor bus voltage and speed as a reference
for maintaining equal output levels, as they
do when operated in isolation from one
another. If, for example, one set operates at
a higher excitation level than the other sets,
the reactive load will not be shared equally.
Similarly, if a generator set is regulated to
a different speed than the others, it will not
share kW load properly with other generator
sets in the system. Each generator set in the
system has two active control systems always
in operation: the excitation control system
regulating voltage, and the fuel control system
regulating engine speed. Generators can be
sharing kW load and have problems sharing
kVAR load, and vice versa. Successful load
sharing requires addressing of both kW and
kVAR load sharing, under both steady state
and transient conditions.
Real power sharing (expressed as kW or unity
power factor load) depends on speed control
and fuel rate control between the generator
sets based on percentage of kW load.
Reactive power (expressed as kVAR or zero

power factor load) is primarily dependent upon voltage
control and excitation system control that is dependent
on the percentage of load between the generator set
 
Addressing Gregorb6 specific question to me:
Electronic components have be known to change in value over time ! Especially if in a marine environment, or maintenance was not carried out over regular periods of time, or because different personnel are used.

By the way... did you carry out the procedure(s) recommended by various forum members ?

Regards, Phil Corso
 
Hello All,

Is there any kind of Load sharing module installed on this plant..

The reason for the same would be cross current compensation /kvar droop compensation scheme to study ..

Maybe onb eof the 2 AVR is not well tuned and send overexcitation on one genset..causing KVAR fluctuations between both Gensets...

Here some notes to take in account :

REACTIVE POWER (KVAR) LOAD SHARING
  • When generator sets operate in parallel the alternator field excitation system of each generator set controls the proportional sharing of the total reactive power requirements (kVAR) of the system.
  • The kVAR load sharing is achieved by increasing or decreasing the field excitation to the systems’ alternators.
  • As the field excitation of one generator set in a group is increased i.e. overexcited it will not lead to an increase in voltage (as it would if it were operating alone) but it will lead to an increase in the proportion of the total kVAR load it will deliver and a decrease in its power factor.
  • As the field excitation of one generator set in a group is decreased i.e. under-excited it will not lead to a decrease in voltage (as it would if it were operating alone) but it will lead to a decrease in the proportion of the total kVAR it will deliver an increase in its power factor.





  • The voltage control system of the generator sets (via the alternator voltage control system) monitors and controls the sharing of the total kVAR load in proportion to the relative rating of the alternators on the systems’ generator sets.


Any time!

Controlsguy25

Thank you Controlsguy25 for an expert information.

Following is submitted:

Yes load sharing scheme is available with the plant, for active power sharing- we are using woodward governor (i couldn't recollect the model) but active load sharing is perfect in the plant, that implies that there is no issue as such with the Governor Droop settings. Further, for reactive power sharing AVR's are used with individual generator having Quadrature droop settings of 2% in both the AVRs (as recommended by the OEM).

when you say "one of the AVR is not well tuned" is it possible that the tuning of the AVR's gets altered overtime. now this was my question- what is the impact of ship's service generator life of the KVAR sharing.

1. Does the reactance value or the resistance value of the rotor/ stator can vary overtime which can lead to unequal sharing of KVARs? if yes is there any reference book wherein i can read about the same.

2. Does rewinding of Stator or Rotor can cause the unequal sharing of KVAR?

3. does variation in he internal impedance of the genset can lead to this unequal sharing.

Thank you
Reagrds
 
Thank you Controlsguy25 for an expert information.

Following is submitted:

Yes load sharing scheme is available with the plant, for active power sharing- we are using woodward governor (i couldn't recollect the model) but active load sharing is perfect in the plant, that implies that there is no issue as such with the Governor Droop settings. Further, for reactive power sharing AVR's are used with individual generator having Quadrature droop settings of 2% in both the AVRs (as recommended by the OEM).

when you say "one of the AVR is not well tuned" is it possible that the tuning of the AVR's gets altered overtime. now this was my question- what is the impact of ship's service generator life of the KVAR sharing.

1. Does the reactance value or the resistance value of the rotor/ stator can vary overtime which can lead to unequal sharing of KVARs? if yes is there any reference book wherein i can read about the same.

2. Does rewinding of Stator or Rotor can cause the unequal sharing of KVAR?

3. does variation in he internal impedance of the genset can lead to this unequal sharing.

Thank you
Reagrds
Hi Shagun Tandon,

I was asking about if Load sharing scheme used /module in KVAR sharing was correct

Did you ever try to compare both Load sharing module behaviour..

Both AVRs should have same caracteristics/settings as you mentionned.

You right to ask those questions regarding generator life service with KVAR sharing...here some answers " written in blue"

1. Does the reactance value or the resistance value of the rotor/ stator can vary overtime which can lead to unequal sharing of KVARs? if yes is there any reference book wherein i can read about the same.

That can be a reason but for me it is relatively minima to give KVAR sharing unbalanced
....
2. Does rewinding of Stator or Rotor can cause the unequal sharing of KVAR?
I am not sure if that got an impact as long as rewinding have been done accoridng to OEM specifications, and rules & regulations..
3. does variation in he internal impedance of the genset can lead to this unequal sharing.
I guess that these generator criterias that you mentionned are more subjet during transients mode/operations

For the internal impedance indeed that can be source of KVAR unbalancing ...
we can speak also about Inertia ( moment of inertia ) of the generator...

Fo sure components life time can be a source of KVAR sharing..

I will have a read on articles about Internal criterias of GENERATOR and associated behaviour..

Can you tell us how you operate thsoe units in the ships..( isochronous/droop mode used)

Also did you have a read on cross current compensation /droop KVAR compensation?

My advice is to get the kvar droop compensation /cross current compensation ok...

Also what is OEM of the Load sharing module ( woordwad ? or other brand)...

A good thing would be to get some datas capture during the mode that you described ..so we can have better overview on the plant behavior then we will be a better support from here..

Any time!
Controls guy25.
 
Shagun Tandon...
The original description of the generators was "Both generators are identical and both using EMRI CAT v2.3 AVRs".

And yes, there will be slight changes in both stator-winding and rotor-winding parameters because of temperature differences ! Even if the stator is rewired! But, those differences are effectively negated by proper tuning.

Why are you of the opinion kVAr (or kW for that matter) must be equal? I agree it's better to have, but unmatched generators are paralleled all the time. What one should be wary of is major circulating-current between them that could cause overheating !

Phil Corso
 
Shagun Tandon...
The original description of the generators was "Both generators are identical and both using EMRI CAT v2.3 AVRs".

And yes, there will be slight changes in both stator-winding and rotor-winding parameters because of temperature differences ! Even if the stator is rewired! But, those differences are effectively negated by proper tuning.

Why are you of the opinion kVAr (or kW for that matter) must be equal? I agree it's better to have, but unmatched generators are paralleled all the time. What one should be wary of is major circulating-current between them that could cause overheating !

Phil Corso
Hello Phil Corso,

Thank you for your comments:

Following is submitted

I do agree sir, that we have unmatched generators are paralleled all the time (with unmatched - i mean to say KVAR sharing) but for a LONG RUN don't you think prolong unequal sharing may lead to reduction in life of the generators (one which is always heavily loaded). Also, what if they start operating beyond their capabilities as defined by the capability curve.

Now coming to proper tuning, may be that is the GOTO thing, but if we have tuned the AVR (in my case COSIMAT N8+) once initially as per recommendation of the OEM, is it even possible they become untuned after several years if yes, then as discussed above (physical parameters) can those be the reasons for it.
 
Hi Shagun Tandon,

I was asking about if Load sharing scheme used /module in KVAR sharing was correct

Did you ever try to compare both Load sharing module behaviour..

Both AVRs should have same caracteristics/settings as you mentionned.

You right to ask those questions regarding generator life service with KVAR sharing...here some answers " written in blue"

1. Does the reactance value or the resistance value of the rotor/ stator can vary overtime which can lead to unequal sharing of KVARs? if yes is there any reference book wherein i can read about the same.

That can be a reason but for me it is relatively minima to give KVAR sharing unbalanced
....
2. Does rewinding of Stator or Rotor can cause the unequal sharing of KVAR?
I am not sure if that got an impact as long as rewinding have been done accoridng to OEM specifications, and rules & regulations..
3. does variation in he internal impedance of the genset can lead to this unequal sharing.
I guess that these generator criterias that you mentionned are more subjet during transients mode/operations

For the internal impedance indeed that can be source of KVAR unbalancing ...
we can speak also about Inertia ( moment of inertia ) of the generator...

Fo sure components life time can be a source of KVAR sharing..

I will have a read on articles about Internal criterias of GENERATOR and associated behaviour..

Can you tell us how you operate thsoe units in the ships..( isochronous/droop mode used)

Also did you have a read on cross current compensation /droop KVAR compensation?

My advice is to get the kvar droop compensation /cross current compensation ok...

Also what is OEM of the Load sharing module ( woordwad ? or other brand)...

A good thing would be to get some datas capture during the mode that you described ..so we can have better overview on the plant behavior then we will be a better support from here..

Any time!
Controls guy25.

Hello Controls guy 25

Following is submitted:

1. Did you ever try to compare both Load sharing module behaviour..

"May i know how can i do it separately for individual generators. we don't have a testing facility onboard as of now."

2. Both AVRs should have same caracteristics/settings as you mentionned.

"Yes sir, as per my knowledge both the settings are same quadrature droop set to 2% and AVR is set for quadrature droop setting not the cross current compensation. (2 AVR's are not communicating to each other)"

3. Does the reactance value or the resistance value of the rotor/ stator can vary overtime which can lead to unequal sharing of KVARs? if yes is there any reference book wherein i can read about the same.

That can be a reason but for me it is relatively minima to give KVAR sharing unbalanced

" Sir, i did a small MATLAB simulation in which i came across that, if the Xd, Xd', Xd" and Xq, Xq', Xq" are varied for a similar rating machine then the reactive power sharing is unequal. (although not much but it is unequal) i don't know the how the reactances vary with time- (i am looking for some reference material wherein i can go in-depth on this topic)"

4. I guess that these generator criterias that you mentioned are more subject during transients mode/operations

"but this is happening during steady state as well"

5. Can you tell us how you operate those units in the ships..( isochronous/droop mode used)

"we operate in a droop mode- The no-load voltage of the incoming generator is made equal to the no-load voltage of the running generator using voltage trimmer in switchboard. Post matching the no-load voltages- the units are synchronised' . once both the units are synchronised we see the KVAR sharing- if it is deviating at steady state then we again alter the trimmer to ensure good KVAR sharing. "

6. Also did you have a read on cross current compensation /droop KVAR compensation?
"yes- after your suggestion'

7. My advice is to get the kvar droop compensation /cross current compensation ok...

"do you intend to say that i need to tune the AVR?" (if yes then why it is required when it is already tuned initially as per OEM recommendation- this is my specific question- i could make out that AVR need tuning but WHY? what factors do change overtime?)

8. Also what is OEM of the Load sharing module ( woordwad ? or other brand)...

"The governor is woodward and the AVR is COSIMAT N8+."

9. A good thing would be to get some datas capture during the mode that you described ..so we can have better overview on the plant behavior then we will be a better support from here..

"i will get back to you on this but for now i have a grab which shows;
Gen-1 380V 258KW 583Amps 50Hz 0.67PF
Gen-2 380V 275KW 520Amps 50Hz 0.77PF

Both generators are running in parallel mode (with APMS- semi Control mode)"

Thank you sir,

Regards
 
Shagon...
Although you addressed James, please advise forum participants if your listed V, kW, A, and PF numbers were provided by instruments or hand calculation? Also, what are excitation Amperes and Volts? Finally, please provide kVAr meter readings and Gen'r Nameplate photo's, if at all possible!
Phil
 
Shagon...
Although you addressed James, please advise forum participants if your listed V, kW, A, and PF numbers were provided by instruments or hand calculation? Also, what are excitation Amperes and Volts?
Phil
Hi All!

No problemo @PhilCorso although it is adressed to me anybody is free to add some notes here to support @Shagun Tandon ...


Also i will review the questions and try to reply as best as i can .

How is frequency maintained? I mean by 2 engines on droop mode are you sure that both should be in droop mode ..How about isochronous mode..
 
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