Automatic Transformer Tap Changer connected to 20 MVA Turbogenerator. Stability 3

[Okpower]

Dear All:

I am considering to connect a 13.8/115 kV transformer to a 20 MVA Gas Turbogenerator (13.8 kV). The turbogenerator has an automatic system to regulate the reactive and active power.
The questions are:
1.- Do I have to use an automatic transformer tap changer to regulate the voltage?
2.- Will I have problems with the stability of the system?

Thanks in advance

 

[crshears]

At first blush I'd say no, you don't have to use an automatic ULTC control scheme.

But I'd go further than that and submit that you would not want to, either; unless time delays or other special features are used to configure the overall control scheme in such a way that the two voltage control elements don't fight each other, I'd just as soon go with manual ULTC control only.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[redlinej]

Will the generator/transformer be connecting to an infinite bus or island?

 

[krisys]

Transient stability shall be a relative phenomenon and hence cannot be ascertained. The steady state stability shall be achieved by the proper engineering.

Since the AVR is a real time continuous controller with PI (proportional and Integral) Controls, it shall be responding to maintain the set voltage at 13.8 kV bus. Where as the OLTC, if set in auto mode would operate to correct the 115 kV bus voltage in a stepped manner. But for the normal operation, it is preferred to keep it in manual.

 

[ScottyUK]

It's quite normal to use the OLTC to flex the reactive despatch of the generator without causing large variation in machine terminal voltage. Tapping down generally increases the reactive export, assuming a high-side tapchanger. The TSO may well have requirements for the OLTC's performance characteristics as part of the connection agreement.

Stability problems are generally an AVR and/or power system stabiliser question. I wouldn't expect the OLTC to cause a stability problem unless for some reason it forced the generator to operate somewhere in the leading region close to the stability limit. It's unusual to see a machine operating in that part if the capability chart, but if the OLTC controller goes haywire it can happen. Master-slave control schemes without a properly defined strategy to handle loss of the master OLTC can result in bad things happening. Don't ask me how I know, I just do.

 

[Okpower]

In fact, there will be two 20 MVA generators in parallel, feeding a 15 MV Petrochemical plant, and exporting the rest of the power to an 115 kV infinite bus.

By now, I guess that I will apply the principle "Less is more".
From your answers, an automatic OLTC may complicate the stability, and may create a problem instead of solving it. As CRSHEARS suggests, I will go just with a manual OLTC.