Paralleling a Diesel Generator Set

[rollingstein]

I have seen many establishments having a backup electric supply using a Diesel Generator Set (AC / 3 phase). e.g. Something like in the Photo Below.

What happens if the connected load grows beyond the capacity of a single generator? e.g. The unit in the photo is rated for 200 kVA. Suppose the load requirement grows can a second similar unit be bought and connected in parallel?

Or do they have to scrap the 200 kVA unit and buy a new (say) 400 kVA unit?

If paralleling AC generators what kind of circuitry is needed?

 

[Doug Huffman]

Reverse power trip, overspeed trip, voltage and current indication, voltage control, phase meter, breaker remote trip and shut.

Start and warmup incoming machine. Bring it to operating speed. Adjust incoming voltage slightly higher. Adjust speed to slightly higher than the running machine - phase meter to show slow rotation in the incoming fast direction. Go to SHUT at "5 minutes before 12:00." Balance real and reactive loads.

 

[zoki85]

What happens if the connected load grows beyond the capacity of a single generator? e.g. The unit in the photo is rated for 200 kVA. Suppose the load requirement grows can a second similar unit be bought and connected in parallel?

Yes. But it is highly recommended that second 200 kVA unit be VERY similar (read "same") to the first one.

If paralleling AC generators what kind of circuitry is needed?

"Circuitry" for synchronisation is a must have.

 

[Doug Huffman]

I think that the machines must be similar only in their speed-droop and voltage-droop characteristics, for ease of real and reactive load sharing. Without, they can still be run together but with more monitoring and manual control.

 

[rollingstein]

The trips I understand. The part I was curious about is the synchronization & voltage control. How are these achieved?

 

[Doug Huffman]

My ship was at La Spezia Marina Militare weapons arsenal during the Yom Kippur War. The base had no shorepower capabilities, so they tried to supply us from a pair of truck mounted diesel electric generators with minimal instrumentation - notably vibrating reed frequency meters for speed indications. They could supply minimal reduced hotel loads with peaks supplied by my battery powered motor-generator sets.

After a few hours the roving electrician announced that their sagging load was due to one diesel's speed drifting and motorizing - reverse powering - the other one, that had a mighty column of smoke and flame standing out of its exhaust stack. We had to run our 1000 Hp diesel - with open pipes, no muffler, for the rest of the visit.

 

[rollingstein]

After a few hours the roving electrician announced that their sagging load was due to one diesel's speed drifting and motorizing - reverse powering - the other one, that had a mighty column of smoke and flame standing out of its exhaust stack. We had to run our 1000 Hp diesel - with open pipes, no muffler, for the rest of the visit.

Great anecdote! Yes, I was wondering exactly about that sort of disaster scenario.

 

[Doug Huffman]

Synchroscope https://en.wikipedia.org/wiki/Synchroscope

 

[Doug Huffman]

That was hardly a disaster, but more of an "Ooops. We can't do that."

Another shipyard supplied alternate power from an old pair of steam turbine generators with cross-connected low pressure steam for load sharing. The powerhouse was taking them off line but forgot to shut the LP steam X-connect before tripping the breakers. The unloaded turbine still with LP steam over sped. The rotor came out of its stator and rattled around the room for minutes before coming to a stop - a huge bundle of conductors like a rat's nest. Four inch thick sections of cast iron foundations were shattered - textbook brittle fracture surfaces. My acquaintance, that was taking log readings at the time, said he just laid down under the biggest machine he could find and waited for the noise to stop.

That was a disaster. It's a wonder no one was killed or even hurt.

 

[rollingstein]

Synchroscope https://en.wikipedia.org/wiki/Synchroscope

Is there a modern digital analogous circuit to achieve this syching automatically? i.e. With a sychroscope someone has to manually adjust the driver right?

e.g. This video of a turbine syncing to grid



If you parallel two smaller Diesel Gensets does an operator have to manually sync them every time you switch to standby power?

 

[Doug Huffman]

I retired in 1995. I'll try to link to a picture of my control panel that I used in the La Spezia story. It has been a bit restored.

 

[rollingstein]

I retired in 1995. I'll try to link to a picture of my control panel that I used in the La Spezia story. It has been a bit restored.

The synchroscopes (like the one shown in the photo below or the video above) they only help you match freq. & phase, correct? Is the voltage matching done separately & prior to using the syncroscope?

 

[Doug Huffman]

Phasing is done during building. The synchroscope shows relative speed/frequency. Yes, voltage is separately regulated and there is no direct reactive power indication in the plant that I operated.

 

[rollingstein]

I retired in 1995. I'll try to link to a picture of my control panel that I used in the La Spezia story. It has been a bit restored.

@Doug

Fascinating photo. What was your role on the ship? Can you elaborate on some of the dials & gauges on there? I'm just so terribly curious.

 

[Doug Huffman]

At that time I was an enlisted nuclear electric plant operator, trained first as a telephone/instrument technician.

The panels, from left to right, are steam plant control dominated by the throttle handwheels, then the reactor control panel and the electric plant control panel. They have been very heavily edited, presumably for security purposes, and I'm a bit hesitant to be too precise. It is amazing enough that this particular ship's equipment was saved.

 

[Doug Huffman]

The real thing un-edited.

 

[rollingstein]

The real thing un-edited.

You should post about your experiences aboard those ships (redacted details perhaps). I, for one, would love to read them. Others too, I think.

 

[rollingstein]

The synchroscope shows relative speed/frequency. Yes, voltage is separately regulated

So, in a diesel genset the relative speed / freq. would be controlled by changing the speed of the prime mover diesel engine? e.g. by changing fuel supply?

How is the voltage fine tuned to match the other generator? Is there a field excitation or something like that that can be changed?

 

[Doug Huffman]

About the DG, yes, running unloaded or solo. In parallel with another machine, torque is load.

About the regulators, yes, they controlled field excitation.

About sea-stories, yes. Read Sontag and Drew's Blind Man's Bluff: The Untold Story of American Submarine Espionage They're good stories but still too classified to risk telling, and particularly our rumors that might leak out.

 

[zoki85]

The trips I understand. The part I was curious about is the synchronization & voltage control. How are these achieved?

Synchronization can be achieved by various means. It is usual that voltages are monitored by double voltmeter, frequencies by double frequency meter; phase matching by a synchroscope or zero-voltmeter or lightbulbs connection capable of withstanding 2x phase voltage. For synchronization 4 conditions to be met:

1. Equality of frequencies
2. Equality of voltages
3. Phase matching
4. Generators masses without acceleration

Difference in frequencies causes a mechanical shock and asymptotic resultant frequency depends on power ratings of generators while electrical shock is due to equalizing current from higher frequency generator's side. Allowed mismatch of frequencies is very small ( like below 1% ). Mismatch of #3 also causes electromechanical shock. Differences should be as small as possible. The shock is capable not only of obstructing synchronization process but under certain conditions generator of much lower power can be damaged by the generator of higher power.

 

[rollingstein]

Synchronization can be achieved by various means. It is usual that voltages are monitored by double voltmeter, frequencies by double frequency meter; phase matching by a synchroscope or zero-voltmeter or lightbulbs connection capable of withstanding 2x phase voltage. For synchronization 4 conditions to be met:

Thanks @zoki!

One point that confused me was that all these descriptions / videos of paralleling a generator or bringing a generator online to grid I've seen show this initial syncing in detail but that's it. Is the system essentially self stabilizing? Why?

i.e. say I match V, f, phase etc. and connect a Generator to grid, won't I then need to keep matching speeds by tuning the fuel to the prime mover or the field excitiation etc? To ensure that the multiple generators stay in lockstep?

In other words, doesn't staying in sync need constant feedback control? From the videos it looks like once you get them in sync you can leave and they essentially stay in sync. Why is this?

 

[zoki85]

If the sync. procedure is followed correctly the system is self-stabilizing. Every sync. machine can work both as generator or motor. For instance, if the synchronous generator connected to the grid completely losses mechanical power supply, it stays locked in sync with the grid and continues to rotate as unloaded synchronous motor. The change of the operating mode is accompanied by the change of the torque angle (angle between rotor and stator fields).