Start up transients in isolated engine-generator group. 1

[xj25]

Hi guys,
I reviewed yesterday a problem in an isolated diesel motor-synchronous generator group ("island" system).
The generator supplies to a compressor and a battery charger.

Basic motor diesel data:
- 800rpm nominal.
- uses a hyrdotransmission to regulate the power to the generator.
- limit speed to disconnect generator: 500rpm.

Generator data:
- Brushless synchronous 400Vrms, 70kVA, 3000rpms nominal, surge current > x2 In,
transient voltage deviation: +/-15% (5% in 500ms) at 100% load on/off
freq.:50Hz, dynamic transients +/-10%

They have problems when the compressor starts from the AC supplied by the generator.

The transient is quite out of specifications (see adjoint). Voltage lowers to 260Vrms!, 141A.
Additionally to this fact, the current drawn by the battery charger have a very strong 5th harmonic.

Questions are:
- In the initial transient, frequency doesn´t seem to chage so much, but voltage so. why?
I forgot quite a lot of sync generators in "island" connection, could appear here the "out of sync" torque angle problem and trip the protections? It seems that the generator keeps supplying the peak start current for quite longer time than usually expected (700ms).

- the battery charger draws a quite high 5th harmonic, could cause any side effects in the generator control or working?

- Sensible solution possibilities (for the start transient) could be use:
1) softstarter, 2) star-delta start 3) ¿bypassable per phase inductor could work?

Regards

 

[itsmoked]

Make sure the compressor is fully unloaded when being started. A bad or mis-adjusted unloader can wreak havoc with a generator. A twenty dollar part can bring it all down.

Perhaps make a change so the charger is not being powered when the compressor is being started. A charger shouldn't care about being cycled on/off.

Yes a soft starter could help a lot. It could also not help a lot. It's rather a crap-shoot since the crazy current waveform it presents to the generator, while reducing the average current, can cause the generator's voltage regulator to do the chicken.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If the motor starts just live with it.
The rule of thumb is that generator capacity should be 300% of motor rating to allow motor starting.
How much are you willing to spend to reduce a voltage dip of less than a second? If you want to spend a lot of money consider a larger generator.
The stable frequency indicates that the prime mover may have the capacity to drive a larger generator.
A PMG (Permanent Magnet Excitation) system may help with motor starting voltage dip.
(Order this as an option when you order a larger generator end.)
The motor starting is drawing the voltage down and that may be dropping the supply voltage to the AVR and reducing the ability of the set to recover the voltage.
When posting problems with motor starting, it may be well to include some motor data. Else we are just guessing. I guess your motor is about 30 kW to 40 kW. Maybe not.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xj25]

Hi,

Compressor motor is about 20kW. I was assuming that "brushless" implied PM generator, but not sure really. I have to review my generator notes...

Good point about the frequency stability and availabe power, that is a good insight.

They can live with the dip, but there are actually more minor loads (lighting and some equipment) that are affected by the transient, that is why I proposed something simple and reliable like a saturable or bypass reactor, to do a voltage-stepped start (cause I am not sure if it is possible to do a delta-star cabling, because of installation restrictions).

Regards!

 

[waross]

A standard set will have an AVR powered from the generator output. When a PMG is added to the outboard end of the shaft, it supplies about 220 Volts 3ph to an AVR designed to work with that supply. The brushless exciter may be the same on both sets.
Before spending big bucks, you may try supplying the AVR from a UPS. If you are suffering from partial voltage collapse that may improve the situation.
The stability adjustment may need to be tweaked on the AVR.
You may consider letting the compressor run and cycling the unloader.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xj25]

Thanks, I actually realised that didn´t remember anything about how a brush and brushless generator works,

Just in case somebody in future lands here:
AVR example Link
Basic topic Link