Load Sharing of Transmission Lines

Yes I agree that the details are inadequate and at the same time it is very difficult to explain without drawing.Any how pl see the details furnished below.

We have got two CPPs with two 220 kV yards with 2X40 MVA 11/40 MVA transformers in each yard.In each yard both 220 kV tfrs are connected to common bus.The 220 kV busses of both yards are connected with each other by a 220 kv tie line.

Confuguration

CPP1 - 60 MW TG connected to 220 kV yard 1 and Line 1 is connected between this yard and the nearest substation.Line length 5.2 KM

CPP2 - 24.5+24.5+10.5 MW TGs connected to 220 kV yard 2 and line 2 is connected between this yard and nearest substation.Length 5.6 KM.Both the lines are coming to our premises in Double circuit towers and getting diverted to yard 1 & 2 in single circuit towers.

The tie line length is 1.4 KM

Line1 ,2 & tie line conductor ,tower ,spacing ,voltage level are same.

Generally CPP 1 internal load will be 47 MW only.CPP2 generation is 34 MW only due to boiler capacity etc and CPP2 load is 41 MW.the deficit 7 MW is drawn from CPP1 through tie line. The Balance 6 MW available in CPP1 is exported to grid.we will always keep the line 1 &2 synchronized with grid and to share the export power equally.

But Line 1(5.2 KM) which is connected to CPP1 is carrying 5 MW load and Line 2(5.6 KM) which is connected to CPP2 is carrying only 1 MW load. If both lines are to be balanced the 3 MW of power from CPP1 has to be shared by Line 1 and 3 MW has to flow through Tie Line(1.4 KM) and to be shared by Line2(5.6 KM)

My assumption is The distance of the tie line + line 2 becomes 7 KM .Hence the impedance of that line is more than the line 1(5.2 KM) and that is why line 1 which is of low impedance compared to Tie line+line2 combined length of 7 KM is sharing more power flow (5 MW) when compared to the 1 MW shared by Line2+tie.

Kindly analyze and clarify.

Although not clearly stated I have to assume that the far end of lines 1&2 both terminate on the same bus at the grid connection. I also have to assume that all four 40MVA transformers are identical ratings/impedance and the taps (if any) are all set the same. Still a few pieces of the puzzle remain, what are the voltages on the three busses, how are they controlled, and how are the VARs distributed?

Although voltage control is important to VAR flow, the thing that controls Watt flow is hidden in the governors and indirectly by the internals of the alternators. It is called Power Angle, which is controlled by the incremental difference in frequency between machines.

For simplicity let's assume that the grid connection is the reference frequency, an OK assumption because we are synchronized to the grid and it is many times larger than your local generators. Since you have excess capacity locally and you want to export it, that means the Power Angles of your machines have to lead the Power Angle of your grid. This is accomplished by increasing the fuel beyond what is necessary for your local use; more fuel in means more power out and the machines are still running at grid frequency. Power always flows from the "faster" alternator(s) to the "slower" one(s).

Now back to your problem, you need to do the same thing locally between your two CPPs (Central Power Plants). You want CPP1 to export 6MW total but then split that power between line1 and line2. You need to adjust the fuel supply such that CPP1 is leading (running slightly "faster") than both the grid and CPP2, while simultaneously having CPP2 running slightly "slower" than CPP1 and "fast" enough so that the excess 3MW does not get consumed locally.

However, without running loadflow studies there is no guarantee that you can accomplish this balancing act. The only other way is to get the cooperation of your Control Operators and the utility and actually tweak the governors (which is the way I've done it, with lots of loadflow studies to back up the actual tweaking).

And don't forget, once you start messing with the Watt loading, the VAR loading and bus voltages will also come into play. If you're lucky enough to get your management to let you run this experiment you will learn more about power system operation than you could ever get in college.

There is another much more costly solution, you could put a phase shifting transformer in your tie bus, but I doubt you want to go that route.

Let us know what happens.

Hi mate

The thickness in conductor size also makes a lot of difference...check that out

Greetings

M & L