Generator Synchronization Procedure

[cloving]

It seems that every generator at the utility that I work for has a different voltage that it must be at during synchronization. Some generators' startup procedure call for the terminal voltage to be less than the bus voltage, while others require that the terminal voltage be greater by a few volts. Why should a generator be synchronized to the grid at any voltage other than the nominal bus voltage? How is such a difference from bus voltage determined?

Thanks
Chris

 

[itsmoked]

Good question! I am also interested in the answer.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[oldfieldguy]

If the generator is very slightly ABOVE bus voltage, you'll go on line exporting power. If it's too far below bus voltage, the bus feeds into the generator, and a sensitive directional (power or current) relay may trip the generator back off-line.

All this presupposes that the calibration is good on your metering, and that you've already done your homework on making sure that rotation is correct on your generator if it's the first try at synching after commissioning or major maintenance.

old field guy

 

[dpc]

Actually the voltage doesn't have much to do with import or export of real power - that is determined by the speed or power angle of the generator.

The voltage relationship between the generator and system controls the reactive power and power factor.

If the generator terminal voltage is exactly matched to the system, it will be at unity power factor when synchronized. If the generator voltage is higher, it will be at a lagging power factor, exporting vars to the system. If the voltage is lower, it will be leading, importing vars.

Except for special circumstances, the generator voltage should be equal to or slightly greater than the system voltage when synchronizing. Since it is typically to pickup some real kW load as soon as synched, the current flow in the generator will tend to cause a voltage drop across the generator internal reactance, hence keeping a slightly high generator voltage usually gives the best transition. I really can't think of situations where it would be helpful to synchronize with a low generator voltage, but I'm sure someone will be along to give some examples. For certain generators, experience may have shown that the transient is minimized by having a slightly low generator voltage, indicating some issues with the accuracy of the voltage measurements.

Basically, the generator voltage should be very close to the grid voltage when synchronized.

 

[cloving]

If the generator goes online above voltage, it is exporting REACTIVE power. Conversely, if it is below voltage, it will consume reactive power, like old field guy said.

I think the idea that we are thinking of is that if the generator is spinning faster (higher frequency) then it will be exporting power when it goes online (real power). This is preferable.

However, I think there is something more to this that we are not seeing. Two coal units in the same plant require different conditions for synchronization. Unit A, the smaller unit, requires that the terminal voltage be less than the bus voltage. Unit B, the larger unit, requires that the opposite. At another plant in the system, gas turbines on the same bus require off-nominal terminal voltage settings.

I am hesitant to believe that it is a function of tap settings on the Generator Step-Up transformers, but thats all that seems remotely reasonable.

 

[ScottyUK]

Hi dpc,

One thing to consider is that at low power and with high reactive import (under-excited) CT phase errors can lead to false operation of the reverse power relay. Conversely these errors can also stop a reverse power relay from operating when the machine is over-excited and motoring. A steam turbine spinning in vacuum takes so little power to maintain sync speed that setting a relay to reliably detect under all excitation conditions it is tricky.

Cloving,

I agree with the consensus above that an incoming generator about to synch to a larger system should be slightly fast and slightly higher in voltage. This ensures that the generator both takes load and is exporting reactive power.

It is absolutely true that the LTC position on the GSU transformer will affect the terminal voltage necessary to achieve synchronising conditions. Why do you doubt this?


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Sometimes I only open my mouth to swap feet...

 

[waross]

It may have to do with the condition of the prime movers.
If a prime mover is getting tired it may be overexcited to export vars and raise the voltage yunder peak loading conditions. I know of one diesel plant that is so old and tired and inefficient that it is used almost exclusively to supply reactive power and control the incoming voltage from a distant power plant.
If new efficient machines with some surplus power are added to the lineup, they may be underexcited so as to not export reactive power and safely push a little extra power out of the machines.
Example. If an old generator is being used as a synchronous capacitor to supply the system KVAR demands, then a generator rated at 0.8 PF may have the real power output (kW) pushed up to the KVA rating, if the prime mover has adequate power.
Or, as has already been mentioned, it may be that a mix of experience and urban legend has shown that the settings are needed to prevent transients from tripping the protection relays.
respectfully

 

[slavag]

Hi.
For prevent of unwanted trip of reverse power protection,
in last projects we use " Wait time " option, that means
status CB close after synch. block for few seconds function 32<.

 

[Hoxton]

Are we getting confused between real power and reactive power?

Generally you should synchronise with the voltages matched, there is then no "flicker" of lighting as you connect. This is as dpc says. In fact a lot of small generators have a "voltage match" feature to equalise the generator voltage with the bus before sychronising.

The different procedure refered to may have grown to compensate for site difference, especially if adjustments are made manually.

Power is another thing. I seem to remember that there is a difference beween steam and hydro turbines, in that one should be synchronised with the speed slightly high, and the other slightly low. I think there can be a problem with cavitation if a hydro unit is connected with lower speed and thus the network drives the turbine momentarily. In most other cases, the speed should be slightly high, so that the unit picks up load, without a potential reverse power.

 

[cloving]

Everyone,
Thank you for your replies and great ideas.

ScottyUK, you are probably right with the tap changer position. Now that I think about it, if the bus voltage metering were anything other than at the low side of the the GSU (I assume it is typically at the high side of the transformer, as is the breaker) then the off-nominal turns ratio would be such that the generator would be synchronizing to something other than matching voltages.

So perhaps, even though the procedures call for the voltages to be different, the voltages really are not different across the breaker.

Chris