Generator Syncing issues, VAR spikes, more

B

Thread Starter

brw

Experiencing a voltage mismatch which two of my units when syncing. We sync those Gen. to our switch-yard breakers on high voltage side of step up. Other two units have gen breakers and sync on low voltage side of GSU's, and never have any problems. Any ideas
 
Unfortunately not much info in your thread to permit one to give you specific answers, however here are some leads to follow:

1. On the generators that are experiencing the voltage mismatch, which voltages does the synchronising device see exactly, i.e.:

a. the busbar voltage and the step-up transformer's high voltage side voltage, or

b. the busbar voltage and the generator terminals' voltage, with correction to transformer vector shift and turns ratio?

2. Is the step up transformer on these generators of the fixed tap type, or variable tap with on-load tap changer?

3. How do you control the generation of reactive power on these generators, i.e. by varying the set-point of the Voltage regulators or by varying the tap of the step-up transformer (in the case the step-up transformer has an on-load tap changer)?

4. By any chance, do the automatic synchronisers of these generators have the same settings as those of the other generators that are working OK?

5. Have you tried doing a manual synchronisation, or simply a manual voltage matching exercise to see what are the voltages required to do a proper voltage match?

6. When you are stating that you are experiencing a voltage mismatch, what are its effects just after synchronisation, i.e. an excessive sudden generation or absorption of reactive power?

7. Are you doing a synchronous or an asynchronous synchronisation?
 
> Has problem appeared recently, or has it been present since commissioning of the two units? <

Since commissioning.
 
Thanks for your reply.

They are experiencing a voltage mismatch between the Gen field volts and bus voltage. The transformers are fixed tap. have to bring the units down to change taps.

We very the setpoints. I believe we have the same set points. We have not tried manual syncing. In reference to after we sync, the Vars go from 0 to a spiked condition, that hangs around for approx 1hour, we are synchronous.
 
It appears that the two source PTs supplying voltage signals to the synchronizer are improperly connected, resulting in an Out-Phase-Closure.

Please contact me On- or Off-List with detail on the GSU xfmr.

Phil Corso
 
Romulo Rodriguez... Typically the same protective relaying job can have PTs with different ratios depending on the primary voltage they are to measure.

Some are connected to read Phase-voltages while others are connected to read Line-voltages!

Regards, Phil Corso
 
Brw,

If your generator is going to full VARS and remaining there for 1hr, my first question is why isn't the voltage setpoint on the generator voltage regulator lowered to obtain the required VARs output, instead of leaving it at full VARs output.

Secondly, (and I am repeating this question) do the synchronizing units of the faulty generators have the same settings as those of the healthy generators?

Did you try to disconnect the close command for the circuit breaker coming from the synchronizer, and connect two voltmeters, one on the busbar VT and the other on the Transformer HV side VT, to see what are the conditions prior to synchronizing?

To Phil,

I am not convinced that this is a question of out-of-phase closure, as:

1. Following closure the generator goes to full VARs for 1hr (well actually why 1hr and no other time is still unclear)

2. A repeated out-of-phase closure will result in either physical damage, of trip due to pole slip.

I am more afraid that they have the same settings for their synchronizers (between the healthy and faulty generators), but the synchronizing units are seeing different voltages, namely:

a. the healthy generators - the voltage of the LV winding of the transformer and the generator voltage.

b. the faulty generators - the voltage of the HV busbar and the voltage of the generator voltage.
 
Jojo, while I agree there are several possibilities OOP is high on my list of for the following reasons:

A) Its relativity easy to trouble-shoot.

B) The problem has persisted from start-up.

C) Voltage mismatch would not produce such high reactive current.

D) High VAr was mentioned, but not high kW. In my opinion this is indicative of OOP closure. Synchronizing torque, kW, is low, but circulating current, mostly inductive, is very high.

d) I chose the GSU as a good place to start because if its vector group is Yd5 or Yd11 (n's omitted for clarity) then there is a plus or minus 30 deg phase shift, that must be accommodated.

E) One can compensate for such phase displacement by using phase-voltage sensing instead of line-voltage. Such a decision is based on the system configuration. (My next request was to have brw to provide a simple One-Line Diagram!)

F) There was no mention of tripping.

G) Circulating current is proportional to the two product of the sources of voltage, but inversely proportional to the total impedance between them. If impedance included a substantial transmission line, then current magnitude could be low!

Regards, Phil
 
Correction to my 20-Oct-09 (12:18) post (my apologies to those uncomfortable w/equations):

H) (Was G) Circulating current is proportional
to the product GEN1xGEN2xSIN[(displacement Angle)/2)], but inversely proportional to the sum of impedances between the two sources, that is, ZG1+ZG2+ZGSU+ZT. ZT represents the transmission-line impedance between the two sources. If total Z is substantial, then the observed "kVAr spike" current might not be enough to initiate a trip!

Regards, Phil
 
Phil,

the point I am banking on is that brw stated that once synchronised, the generator goes to full VARs and remains there for a good 1hr. so I am not seeing a question of OOP synchronisation, but rather a voltage mismatch (voltage regulator too high setpoint) which is going uncorrected, both prior to synchronisation and after. I am afraid that what brw is calling a spike is actually an uncontrolled overshoot to full VARs once the generator CB is closed.

An OOP closure will cause a surge on the system, as you stated depending on system impedances, but after a couple of seconds should settle to an acceptable VAR value, once the rotor oscillation dampens out, and not remain there for 1hr.
 
I think the term spiked condition, may be taken as maxed. It goes really high comparable to the other two units, An hour is variable as well. I sit around the other night and witnessed the condition, on one of the units and it was approx an hour that time. I haven't gotten a straight answer with operations why there is no attempt to adjust. They don't use Var control here. My PI data unfortunately is incomplete. I haven't been here long so I am going to work on that. My steamer unit, has lost a few switch-yard breakers(the one's it syncs to), about every 1.5 years or so, a pole is lost. there is definitely a break down happening. I am grateful for the responses, keep them coming, they are definitely helping me get my head around it better.
 
Brwtx,

Now you have given another piece of valuable information, your plant is destroying a switching unit every 1.5 years. Well things are definitely wrong at your end as:

1. synchronization should be smooth, and not cause damage. I am afraid that your generator might be incurring far worse damage than destroying a switching unit without anybody really appreciating it.
2. no action is taken by the operations staff in reducing VAR output from the generator after this goes to max condition, for no known valid reason (obviously from their end).

I would recommend start afresh, assume that this generator is going to be commissioned the first time, and do all the checks and adjustments associated with commissioning such generators. Don't skip steps, and do it as early as possible before you end up without this generator. Make sure you have all the drawings 'as fitted' before you start.

One last question, how is the busbar/line voltage control performed, if as you stated earlier you do not have variable tap transformers on your generators and VAR control is not used at your plant? This question also relates to the control of the voltage to your auxiliary plant.
 
I apologize for occupying everyone's time on this issue, and I appreciate all information and advice. I found the discrepancy to be coming from some pt's installed at star up. They are dragging the voltage down, approx 4%. Essentially making the reading 4% off. During auto sync the exciter matches perceived voltage, thus syncing with a 4% voltage mismatch. That is a lot of reactance. Thanks again everyone.
 
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