Signal Receiving Relay 85T and OGS

The distance between TJ and UN 500kV sub-station is 134km. It has two lines which each conductive capacity are 2400A but for safety reason load of conductor maximum is 80% from conductive capacity, so that 2400A * 80% = 1920A let we say 2000A.

The TJ Sub-Station is connecting between 4 power plants to 500kV line grid. Each power plant capacity is 660MW (4 x 660MW = 2,640MW) and current line can be calculated as [(2,640,000kW / 500kV)]/2 = 2,640A

It is over limit of line capacity.

The solution is using two lines of Interconnection bus transformer (IBT) to 150kV by 4,000A deliver to 500kV (@ 2000A) and remain 1280A deliver to 150kV (@ 640A).

If one of two line of 500kV get trouble, they need new scheme to avoid over capacity on 500kV line that we call OGS (over generating load shedding) system. For example if CB line #2 at UN sub-station get trip, the relay 85T will sent to TJ sub-station to open CB at TJ side line #2 but line #1 will over capacity.

How OGS (over generating load shedding) system protection work? If the line current is over than setting (at line #1), for example 2400A, one of four power plant will trip.

The relay 85T only order to open CB at TJ sub-station to line #2 side and OGS for protection at line #1 from over capacity.

Relay 85T is enough if total unit only two (2 x 660MW) but if total unit add become 4 x 660MW and 500kV line still two, the OGS system protection is mandatory.