Generator Frequency

If you are synchronised to the grid and the grid is still on, having isolated the fault, then you can not have any problem of generator frequency. The grid controllers will maintain the frequencies.

However if you are disconnected from the grid due to the fault, then obviously the frequency is likely to go a bit high till your governors take over and put it back to the approximate frequency.

It depends entirely upon the type of fault you encounter. If the fault is low frequency, then your generator(s) will be going slow and the frequency will be low.

If the utility separates from a substation miles away that your plant is connected to, the frequency, load and voltage on your generators will vary with the load applied to them.

In one instance I was present at a plant which was in co-generation operation and when a substation a few miles away lost its connection to the utility, the plant where I was working went under frequency and to maximum load on all six of their generators. The utility tie to the plant's substation did not immediately open and the plant supplied power to the down town area of a fair sized town for about five to ten seconds. This tripped some of their boilers due to low drum level as the pumps were unable to keep up with the demand. The problem continued until their local tie opened due to under frequency.

When the tie opened, the remaining generators were able to stay on line, recover and maintain frequency keeping the power house and associated plant up and running.

In a situation at another location, a trash burning plant miles from the nearest substation had a problem with overspeed trips when they lost their connection to the grid due to an unsupervised breaker opening miles away. Their local tie was not opening but the distant substation was loosing its tie and leaving the plant with their local tie still closed and no load. The problem was due to trees touching the power lines during wind storms. We had to cook up a scheme to detect the loss of tie by monitoring the frequency (machine speed) for them.

So, a better explanation of the problem is in order if you expect a usable answer

If the utility generations trips, and you don't, your frequency will go down for a bit as your generator assumes all the load. Your under frequency relay (if you have one) will trip you off before your generator grinds to a halt (you hope). Reverse power? I don't think so unless you dial your speed (frequency) down while you're still in parallel with the utility. Oh yes, over speed (frequency) will occur if your unit trips while fully loaded (unless the governor can grab the fuel (steam, etc.) valve in time). I hope this helps.

If the local machine is exporting power to the utility, the frequency will go up when the tie opens. If the local system does not switch to frequency control when the tie is lost, the machine speed will stay higher than synchronous speed, 50/60 Hz. It may even overspeed and trip if the governor and valve controls are not fast enough.

If the AVR does not swich from KVAR or load control to Voltage control, the voltage will go high and stay high also.

As the old saying goes, "This door swings both ways."

Good post Turbo! I think the voltage factor was overlooked, you're right, usually if a unit(s) is operating in parallel with a utility it will be in a var sharing mode. The AVR must control the voltage just as the governor controls the frequency after a utility trip. Thanks!

hughes838

a reverse power relay should operate only when the generator is motoring ie when it is no longer generating power.in marine systems it may happen when there is a prime mover shut down for any reasons and the breakers are stll closed with the bus bars.a fault in bus bar (utility grid)cannot reverse the power of generator.