GSU Tap changer

[Power0020]

Checking the power flow on a GSU transformer with the primary connected to 15.75 kV generator and secondary connected to 220 kV (LTC: 4 x +/-2.5%), at rated generator power @ 0.85 lag p.f., the power flow math. model shows a primary required voltage of 1.1 p.u. (which is greater than 105% of generator o/p).

the power transfer is simply P=V1*V2*Sin (Delta) / XT, with V1 & V2 primary and secondary voltages (p.u.) respectively, Delta is the differential angle and XT is the transformer impedance. Also, V1 = V2+I.XT

This means that such power isn't be transferred through the transformer @ 0.85 lag p.f., but may be transferred at 1 pf. The tap changer (installed on 220 kV side) will act to keep the primary voltage at 15.75 kV but this will lead to reduced power flow!, limiting the GSU capability.

One solution to this problem is to reduce the GSU impedance, but this is not feasible as the IEC 60076 recommends Z >12.5 % for ratings 100 MVA and above.

I wonder if I increased the tap changer range, will this help the power transfer capability? what will change in the power equation above?

 

[waross]

Maximum power transfer is limited by the transformer KVA rating, or more specifically by the maximum current rating.
Real power transfer depends on the phase angle between the generator and the grid. Real power transfer is increased by advancing the torque output of the prime mover.
The transfer of reactive power depends on the respective voltages of the generator and the grid.
If you advance the tap changer so as to increase the voltage at the 220kV side with a constant generator voltage you will increase the flow of reactive power.
Anecdote. I encountered a situation where a small hydro plant could only operate at reduced output due to a combination of utility requirements and line impedance. The utility specified limits of voltages and power factors had the effect of reducing the allowable output of the generator. The solution was to negotiate relaxed operation parameters with the utility. I left the area shortly after and I don't know if the negotiations were successful.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Power0020]

Thanks waross, I am facing the same problem with a GTG, the proposed GSU can't deliver rated turbine power at 0.85 p.f. lag as the developped voltage at generator terminals will exceed +5%

Would installing a tap changer on the generator side help this? I don't think so as the impedance will stay the same (very small variation).

 

[ScottyUK]

Put the tapchanger in manual control. Use it for coarse control of your reactive despatch. Allow the AVR to regulate the machine terminal voltage, not the OLTC.

You have nowhere near enough taps for a GSU transformer of that size, and the steps between taps are too great.

 

[waross]

I'll describe a sequence of operations, not as a suggested operating procedure, but as a way of explaining the limits you are facing.
Imagine the GTG on-line and operating at unity power factor. Now increase the fuel delivery until the unit is delivering 100% power at unity power factor.
Now, increase the excitation level so as to produce VARs.
As the excitation is increased and the power factor drops, the voltage will be increasing. Eventually you will reach the +5% Regulatory voltage limit. You are at the mercy of the distribution line impedance.
I imagine that you are on a distribution circuit.
I doubt that you can do anything about the line impedance and if the line is feeding other consumers there won't be much forgiveness for the 5% over voltage limit.
Time to seek a political solution to a technical problem.
If the utility needs your energy you will be in a better bargaining position.
You may try to have the .85 PF requirement relaxed. Be aware that the grid needs the MVARs. If you are allowed to generate at a higher PF, then other machines on the grid must supply the MVARs. This may reduce the MW capacity of some of the utilities generation.
While I don't know what size machine you have, I will use 100 MVA as an example.
100 MVA @ 0.85 PF =

MW 85​

MVAR 52​

At 100% MW output the utility wants to see 52 MVARs exported. Explore the cost and consider offering to supply the required MVARs at other places on the grid with capacitor banks.
For example, if you use capacitors to supply 20 MVARs to the grid back at the substation, then you would need only 32 MVARs from the GTG.
You could now run at a PF of close to 95%. That may allow operation within the +5% voltage limit.
I would suggest offering to pay for the supply and installation of the capacitors with the maintenance becoming the responsibility of the utility.
You are being limited by the impedance of the distribution line. Changing the location of the tap changer or the impedance of the transformer will have no effect.
Good luck with negotiations.
Keep us informed.

Bill
--------------------
"Why not the best?"
Jimmy Carter