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About Transformer Rectifier Units (28 V DC)

Posted: Fri Jun 26, 2020 9:50 am
by Amateur
Hi,

Since the fifties (some sources mention the brittish Bristol Type 167 Brabazon of the forties), commercial and military transporters have used electrial power systems based on 115 V AC three phase 400 Hz, rectified to mainly 28 V DC (and later to 270 V DC, according to MIL-STD-704). The variances of the frequency (nominal 400 Hz) are only allowed in the interval of 393...407 Hz (steady state), according to Table 1, Mil-STD-704 F, Rev 1991.05.01. But these TRU:s don´t have regulation, so when you increase the power level, the voltage level drops. But the IDG:s of the engines guarantee a very precise voltage regulation on the primary side of the TRU.

Boeing 787 has a "new" type of generators, where the frequency is “wild” (nominal 400 Hz but with a variancy of 360...800 Hz, corresponding to Table 2, MIL-STD-704 F). The nominal voltage level is 230 V AC rms, twice the “military” level.

Now to my question: is a new, regulated TRU (input three phase 115 V, output 28 V DC) necessary if you have a “wild” generator? If the frequency is varying so much, it must affect the stability of the voltage level (?). In the TRU, you don´t have regulating Power Factor Correction ("PFC") but only a “passive” three phase transformer (delta and star windings on the secondary side), and an Interphase transformer (to equal the two current of the two secondary windings) and rectifiers.

The French company Thales markets since a couple of years a new TRU with regulation, called “RTRU” (= Regulated TRU, max power 6 300 kW). The American company Crane has for a long time produced the regulated TRU Model 81-012 (max power 11,2 kW). Are these regulated TRU:s just an “add-on” or are regulated TRU:s (RTRU:s) necessary for a 28 V DC system? Table IV indicates for a 28 V DC line a max variation of 22,0...29,0 V DC.

The 12- pulse rectifier has a very high reliability since there is no electronics in these devices. Since the new RTRU:s are regulated, they are also containing complex control electronics (= more components), high frequency semiconductors and sensistive capacitors (very temperature sensitive, especially electrolytic capacitors) and a fan, they are all affecting the MTBF in a negative way. But is this difference “acceptable”? And if so, who regulates the MTBF of TRU:s/RTRU:s?

(Normally, MIL-STD are only used for military applications but since I don´t have the standards for civilian aircraft, I used the DoD MIL-STD).


Regards

Re: About Transformer Rectifier Units (28 V DC)

Posted: Fri Jun 26, 2020 12:53 pm
by Dalmd88
The new VFSG really are not that new. The MD90 used this 'wild' system from the start. I never played with them much when I was in the hangar or line mtc. I do recall the system had some big teething problems when the plane was first introduced. I think this system was the most problematic of the MD90 and contributed to it's precised poor reliability for dispatch. With time I thing the system became very reliable and that is one of the reasons Boeing installed it on the 787.

I seem to recall one of the big advantages was weight of the VFSG is much less than the IDG and I think the drag on the engine is less.

Re: About Transformer Rectifier Units (28 V DC)

Posted: Fri Jun 26, 2020 1:42 pm
by FlyHossD
Dalmd88 wrote:
The new VFSG really are not that new. The MD90 used this 'wild' system from the start. I never played with them much when I was in the hangar or line mtc. I do recall the system had some big teething problems when the plane was first introduced. I think this system was the most problematic of the MD90 and contributed to it's precised poor reliability for dispatch. With time I thing the system became very reliable and that is one of the reasons Boeing installed it on the 787.

I seem to recall one of the big advantages was weight of the VFSG is much less than the IDG and I think the drag on the engine is less.


Yes, this isn't new. Off the top of my head, I recall that the ATR series used AC "Wild" generators for high draw items like prop heat and window heat(?) and that the DHC-7 (and -8?) used "Variable" AC generators. In both case, I believe it was the Normal AC system that had TRUs to convert to DC power (not the AC Wild or Variable AC). It has been many years, though, so maybe my memory is flawed...

Re: About Transformer Rectifier Units (28 V DC)

Posted: Fri Jun 26, 2020 6:32 pm
by txjim
A couple things to consider regarding diode rectifiers:
- In my experience (mostly MIL) you are trading complexity for heat and weight. There is not necessarily an increase of MTBF as I've seen rectifiers fail, even in units certified against DO-160.
- Depending upon the topology and construction, rectifier units can get pretty loud.

Re: About Transformer Rectifier Units (28 V DC)

Posted: Sat Jun 27, 2020 12:22 am
by LMP737
Dalmd88 wrote:
The new VFSG really are not that new. The MD90 used this 'wild' system from the start. I never played with them much when I was in the hangar or line mtc. I do recall the system had some big teething problems when the plane was first introduced. I think this system was the most problematic of the MD90 and contributed to it's precised poor reliability for dispatch. With time I thing the system became very reliable and that is one of the reasons Boeing installed it on the 787.

I seem to recall one of the big advantages was weight of the VFSG is much less than the IDG and I think the drag on the engine is less.


The MD-90, what a pain that aircraft could be. Good for OT though. The electrical system was way too sensitive and latch onto faults like nobody's business.. often times you would have to power down the entire aircraft to clear it. For awhile we would fire up the APU before the first flight of the day to avoid gate calls. Plane got better a little better as bugs were worked out and maintenance along with flight crews gained more experience with it. And then AA returned them to the leasor, lol.