Parallel operation of generators

[172_driver]

Hi Technicians & Engineers!

I've read the ATPL theory but still have some question marks regarding electrics that I am sure you can help me with, if you don't mind.

What I am wondering about is operating generators in parallel. The question is basically, is my reasoning in this matter correct?? I think it sounds logical to me but would be interested to hear what a "professional" would say about it.

I understand, to parallell the generators they have to be EXACTLY in phase and same voltage + frequency has to be the same so they don't drift out of phase. This is magically taken care of by the CGU that auto-synchronises.

Now when the GCU has closed the GCB and both (or more) generators are connected together... both generators must remain at EXACTLY the same frequency. Any tiny difference, even 0,1 Hz, will have the generators to drift out of phase, the peak voltage will occur at different times and there will be circulating currents between the generators.

This is taken care of by the "load controller" - that eliminates the problem with circulating currents and keep equal load sharing by keeping the sine waves in phase. If the generators go slightly out of phase, the generator that "leads" will peak first and take a slightly higher load (more amperes). This momentarily higher load from the "leading" generator is sensed and sends a correction signal to the speed governor of the other generator to speed up.

So load sharing - basically achieved by keeping the sine waves in phase on all phases A, B, C??

Thanks in advance!

[Blacksheep]

You've answered your own question pretty well.

Have you thought about writing a text book?

[SeldomFixit]

115V plus / minus 5v
400Hz plus / minus 5 Hz
Phase rotation correct
Oncoming Gen " toggled " to achieve the match
Real load in KV's and Reactive loads in KVA's looked after by GCU's & Load Controllers
Faults on the Sync BUS will trip the BTB's
Faults on the load BUS will trip the GB - if the GCR trips, then the GB will almost certainly trip.

[mitzy69]

gens in parallel

one gen will drive the other gen so they stay at same frequency
so the load on the drives will be different, and controlling the drives will mean the load on both gens the same.
controlling the field will control volts.

[172_driver]

Blacksheep, cheers!

Thanks for your inputs. However, fault on a load bus wouldn't it be appropriate to trip the BTB as well so the other generator(s) doesn't feed this "empty hole"?

Mitzy69, I see your point that the generators also help keeping themselves synchronised. The circulating current will drag the lagging generator along. Does this principle apply to stationary armature (pick-off coils) alternators as well?

[Avtrician]

To be pedantic, what you have described is the Paralleling of Alternators, which produce AC. Generators produce DC.

The ground techs (Avionics/Electrical) will Parrallel the Gens on a ground run. This entails adjusting the regulators for the same Voltage out put, thus when both Gens are selected on they will share the load evenly..

Alternators once the frequency and phase is matched, will lock each other into sync, unless one of the has a mechanical problem and then it will drop out.

A very simplified explanation, tis many many years since I have had to do either.

[Bus429]

Flashing the field

As well as doing paralleling checks on AC & DC systems, I remember having to flash the field on a single engined...P47! The voltage regulator was mounted on the left firewall (engine side) so it was a case of cowls off, wedge self firmly against leading edge and hope the guy in the cockpit was on the ball!

[aveng]

Thanks for your inputs. However, fault on a load bus wouldn't it be appropriate to trip the BTB as well so the other generator(s) doesn't feed this "empty hole"?

sounds like a 727 elec system?

If one of the load generators gcb trips - the other 2 gens would now be supplying that gens bus (and there own) via the bus tie - not a black hole! The hole idea of a tie bus is not to lose power unnecessarily.

[172_driver]

Hi, sorry.. I meant an earth fault on a load bus. Personally I don't exactly know what could cause an earth fault, a loose wire perhaps?? Someone with an example??

[Bus429]

Hi, sorry.. I meant an earth fault on a load bus. Personally I don't exactly know what could cause an earth fault, a loose wire perhaps?? Someone with an example??

How about a feeder chafing to...ground (airframe)?

[Piper19]

The 727 had parallelling, but the crew had to do the frequency adjust manually (if I remember well, 727 too long ago).

The 777 again has parallelling during power breaks. It occurs when you swith power sources between external, APU and generator power on ground, to avoid the flight deck screens go dark and other system loose power for a moment.
A reference frequency circuit is in the BPCU, and for a moment two sources are parallelled. The generator controllers adjust IDG speed to match power of the other source, for the APU it's the APU RPM that is adjusted through the APU controller.

[172_driver]

Once again, thanks for your answers. All inputs are greatly appreciated.

[spannerless]

Electrical theory

172 Driver

There plenty of electrical theory books around some are written in laymans terms too.

However, one of the most popular books still in print is:-

Aircraft Electrical Systems (Hardcover)
by E.H.J. Pallett (Author)

3rd edition

Tisn't cheap mind!

[Swedish Steve]

The 777 again has parallelling during power breaks.

So does the A330, it lasts for a fraction of a second (180ms rings a bell), but only on the ground. The A320 does not. On the A320 there is a clunk as you change sources, but doesn't cause any problems.

Anyway, what I want to know is..Why did old aircraft have continuous paralleling (Trident Tristar), when it is not allowed today?