Wrong generator synchronization ? 1

[edison123]

Recently, a large hydrogenerator (25 MW, 14 Poles) was wrongly syncronized by a jittery operator. The bus frequency was 50.3 Hz and the machine's was 47.85 Hz (as per data recorder) when this happened. The stator current (3 KA) went up by two times the rated current (1.5 KA) and heavy grunt was heard from the machine. The machine tripped on under impedance relay. No synch check relay (which would have prevented such a out-of-phase synchronizing) is present in this unit.

How bad is this synchronizing (47.85 Hz to 50.3 Hz)and what are the effects of such a bad synchronizing ?

What checks should be made on the machine after the event ? (like open up and check all the fixing bolts/keys on the rotor, check the stator frame welds for any cracks ? Do a hipot ? etc.)

Any such experiences and ideas shared would be welcome.

 

[Skogsgurra]

I had a similar experience during my apprentice days. I was left on duty in the Jarpstrommen hydro-electric power station with three 40 MW vertical machines. I think very similar to what you have. Not so sure about number of poles, but speed was something like 200 RPM, so it must have been close to your machine.

Anyhow, there was a "synchronoscope" and a synch check relay. So nothing should go wrong. Right? Wrong!

The operators were so used to the 1 second delay in the breaker (a 1942 installation) and were very particular to have everything just right before pressing the button. Especially the frequency difference should be as low as possible. I had been told so, but not why it was so important.

Then, I had a phone call from Norway. They wanted me to bring up and connect one of the machines. I got it up in speed. I got the voltage right and I pressed the button.

The machines were 96 metres down and nothing but granite and an elevator shaft between me and machines but I heard the "thud" and I certainly saw the alarm light coming on. And all other blinking lights and the phones...

What happened? I had not paid enough attention to frequency. There was a smal difference - but big enough to make the two systems drift away almost 180 degrees after the synch check relay had said OK. So, the connection happened almost, but not exactly, out of phase. Tripping most protections there were.

The boss arrived and told me several things that I didn't want to hear. Then, he just reset everything and put the machine into service. No worries about damages. I never thought that an inspection should be necessary. It is only when you bring the subject up that I think that we should have done some inspection before starting again - but we didn't. And the machines still work. After more than forty years. They were old ASEA (ABB) machines and I think that the low speed takes a lot of the inertia forces out of the happening. SC current forces had been taken into account when designing the machines. They had been factory tested with short circuit under full speed and full exitation.

Gunnar Englund

http://www.gke.org

 

[edison123]

Thx Skogs. As always, you are a comfort.

The problem I face is a beating (or call it flapping) noise (at twice the rpm) from the unexcited dc exciter armature whose source could not be pinned down as of now.

Will let you know how this goes.

 

[dpc]

I had a similar situation with a smaller hydro unit. The machine wasn't noticeably damaged, but the foundation cracked. Of course, the concrete/grout was 80 years old. You might want to take a look for cracks around the anchor bolt area.

 

[jplinn]

You should inspect your damper bars. During an out of syncronization large currents can flow in the damper bars.

I have inspected several units that have been closed in out of synch and on both I found burning on the damper bars.

In addition, you should check the slip rings. During a out of syncronization event the generator shaft can bounce. During the bounce the slip ring brushes are thrown off of the slip ring in non-uniform pattern. Resulting in one or two brushes carring all of the excitation current and burning the slip ring.

Large forces will also occur on the end windings pf the the stator. The coil ends should be checked for damage especially knocked loose blocking.

During startup watch the bearing temperatures.