Power Plant Blackout After Load is Rejected

M

Thread Starter

Maaz Naseer

In power plant having 4 generators:

gas turbine 1 and 2 (35 MVA each)
gas turbine 3 (10 MVA)
steam turbine (20 MVA) running on exhaust of gtg1 and gtg2

plant has a load of 60 MW and also supplying 15 MW to the utility through three feeders. complete blackout is observed when a load of 35 MW is tripped due to a fault. After that outgoing feeders are tripped on over current, gtg2 is tripped due to reverse power and finally a complete blackout due to over frequency of the system.

gtg2, gtg3 and stg operates in droop and pf control mode while gtg1 is in iso and voltage control mode.

1. What are the possible solutions to keep the system operating after the disconnection of 35 MW load?

2. i think if one generator is removed from the system (gtg2), then the system will be stable. is this possible to suddenly open the CB of generator when it is operating at full load? what are the consequences of doing this on turbine?

Thank you in advance
 
Maaz Naseer,

Over-frequency occurs when there is too much generation for the existing load. So, the best cure for a problem like this is a power management system which will very quickly reduce the load of the droop machines while keeping the Isoch machine above the reverse power setting.

Load rejection is the term for opening the generator breaker while the prime mover is running. And, it's not so bad for a gas turbine (because the gas turbine effectively has a big "brake" on it: the axial compressor which consumes as much as 2 of every three horsepower produced by the turbine). And, if the gas turbine control system is properly tuned, then it will not lose flame on a load rejection (but that takes testing and tuning).

I've never seen a site that is supplying power to an external grid that has any units operating in Isoch speed control when connected to the grid. Unless there is some external load control, or power management system, that is controlling the Isoch unit it wouldn't seem possible to have an Isoch machine connected to the grid. So, it seems there's something about the plant configuration we haven't been told.

The Isoch unit can only reduce it's load to a very slight reverse power before it will be tripped. And, if there are Droop machines operating at a high load when the event starts and continues with the Isoch machine the Isoch machine <i>should</i>try to reduce it's load--but it can't go much below zero MW before it's breaker will be opened on reverse power. If, after that the Droop machines are still on-line with a load that is much less than they are trying to produce the frequency will increase and that will likely lead to an over-frequency trip.
 
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