Tech Explanation on Diesel Engine in AC Generator Set

[IOC-AUS]

Hello All,
I was hoping to get a Technical Explanation on the Power ( kW Input ) requirements of an Industrial Diesel Engine 1500rpm in a 4 Pole AC Generator Set.

Is there any impact on kW Rating ( or another way to ask kW Efficiency ) of a Diesel Engine when the Output Supply and AC Alternator configuration are different.

for example,
● CASE # 1 - A Diesel Engine is used in a Diesel Generator Set configured to Supply 3 Phase 4 Wire Output 415V 50Hz ( typically the AC Alternator is connected in Series Star Cofiguration ).
● CASE # 2 - The same Diesel Engine is used in a Diesel Generator Set to Supply 3 Phase 4 Wire Output 690V 50Hz ( typically the AC Alternator is connected in Three Phase Zig Zag Cofiguration ).

I understand the Current Rating will be lower on the 690V Supply compared to 415V Supply.

Not sure if the AC Alternator Efficiency will be different for each Winding Confguration but I would assume the Efficiency should be in the order of 92 to 94%.

Welcome any comments RE this enquiry that may help understand the Diesel Engine kW Rating within each System.

 

[LittleInch]

Errr, some idea of the power required in each option / amps generated and the rated power for the diesel engine at 1500 rpm might be useful.

 

[IOC-AUS]

Yeah good point.
Load will be a 400 kW Electric Motor with VFD Driving a Water Pump.
690V Motor - Full Load = 450 Amps per Phase.
● Case 1 - Power Draw 347 kW ( 1018 rpm )
● Case 2 - Power Draw 370 kW ( 1025 rpm )
● Case - Power Draw 395 kW ( 1034 rpm )

 

[waross]

First, the engine develops enough power to support the load plus losses.
The engine is not always loaded to 100% of rated power.
Each winding will have a current rating, and a voltage rating.
The maximum burden in KVA that the generator will supply is Max Volts times Max Amps time Root 3 divided by 1000, or A x V x 1.73 / 1000. = KVA Max.
It is common practice to supply enough kW to support 80% of the rated KVA.
So, for example.
Winding Max Volts = 240 Volts.
Winding Max Amps = 104 Amps
6 Windings, 2 per phase.
Max KVA = 240 V x 104 A x 6 x / 1000 = 150 KVA.
For standby service, this would be driven with an engine rated at 150 KVA x 0.8 PF = 120 kW.
For prime power service it is typical to over-size the engine by 10% (132 kW) so that after 15,000 hours, the engine will still be able to output 120 kW.
Let's stick to standby sets for now.
The different voltages are realize by series or parallel connections, star or delta connections and voltage regulator adjustments.

Example #1, Terminal voltage = 415 Volts,
Connection #1, series delta, 208/415 Volts, center tapped.
Voltage regulator set to 208 Volts.
KVA = 208 V x 104 Amps x 6 / 1000 = 130 KVA Max.
120 kW / 130 KVA = 0.92 Allowable PF.

Example #2 Terminal voltage = 415 Volts
Connection #2. Parallel star, 240/415 Volts.
Voltage regulator set to 240 Volts.
KVA = 240 Volts x 104 Amps x 6 windings / 1000 = 150 KVA
120 kW /150 KVA = 0.8 Allowable PF

Example #3 Terminal voltage = 690 Volts.
Connection, series star, 400/690 Volts
Voltage regulator set to 200 Volts.
KVA = 200 Volts x 104 Amps x 6 windings / 1000 = 125 KVA

Generator ends are rated in KVA.
Rated Maximum Allowable KVA = Winding Rated Voltage X Winding Rated Current X Number of Windings / 1000.
Actual Maximum Allowable KVA = Winding Actual Voltage (Voltage Regulator Setting) X Winding Rated Current X Number of Windings / 1000.
Note #1: When using winding values to calculate KVA, the root three factor is replaced by the number of windings.
Note #2: Both kW and KVA must be considered when sizing a gen-set.
The set must be capable of delivering the required current at the rated voltage (KVA)
The prime mover must be capable of meeting the required kW demand of the load.
If the load PF is above 0.8, the set must be oversized relative to the KVA rating.

 

[IOC-AUS]

Hello All,
I am still wrestling with this enquiry and was hoping for some Tech Calculations to verify what we are thinking ( validate our maths ).
● Load = 400 kW Electric Motor Driven Pump ( Water Pump ) with VFD
● Client has advised Max FLC = 450 Amps per Phase ( I assume this is @ 400 kW )
● Duty Point # 1 = 395 kW ( Client has advised 1034 rpm Speed via the VFD )
● Alternative Duty Point # 2 = 370 kW ( Client has advised 1025 rpm Speed via the VFD )

We are sizing a Diesel Generator Set and considering Harmonics / Waveform Distortion we had calculated the need for an 800 kVA AC Alternator ( Generator End ).
Now looking at Diesel Engine requirement @ 1500rpm ( 4 Pole ) to suitably Start and Run this Pump Motor.

Is this enough information to calculate / validate a suitable Diesel Engine kW with suitable Oversize AC Alternator ( Generator ) to match.

We are considering AVR Control 4 Pole Brush less AC Alternator which will be around 88% Efficient.

Any Calculations and advice is much appreciated.

 

[IOC-AUS]

Hello All,
I am still wrestling with this enquiry and was hoping for some Tech Calculations to verify what we are thinking ( validate our maths ).
● Load = 400 kW Electric Motor Driven Pump ( Water Pump ) with VFD
● Client has advised Max FLC = 450 Amps per Phase ( I assume this is @ 400 kW )
● Duty Point # 1 = 395 kW ( Client has advised 1034 rpm Speed via the VFD )
● Alternative Duty Point # 2 = 370 kW ( Client has advised 1025 rpm Speed via the VFD )

We are sizing a Diesel Generator Set and considering Harmonics / Waveform Distortion we had calculated the need for an 800 kVA AC Alternator ( Generator End ).
Now looking at Diesel Engine requirement @ 1500rpm ( 4 Pole ) to suitably Start and Run this Pump Motor.

Is this enough information to calculate / validate a suitable Diesel Engine kW with suitable Oversize AC Alternator ( Generator ) to match.

We are considering AVR Control 4 Pole Brush less AC Alternator which will be around 88% Efficient.

Any Calculations and advice is much appreciated.

Sorry this is 690V Electric Motor.

 

[MintJulep]

The only load is the pump?

Why not just drive the pump directly from the engine?

 

[waross]

Why not just drive the pump directly from the engine?

If the engine HP is rated at 1500 RPM, 1800 RPM, 3000 RPM or 3600 RPM you will take a significant HP hit at 1034 RPM.

We are sizing a Diesel Generator Set and considering Harmonics / Waveform Distortion we had calculated the need for an 800 kVA AC Alternator ( Generator End ).
Now looking at Diesel Engine requirement @ 1500rpm ( 4 Pole ) to suitably Start and Run this Pump Motor.

Is this enough information to calculate / validate a suitable Diesel Engine kW with suitable Oversize AC Alternator ( Generator ) to match.

We are considering AVR Control 4 Pole Brush less AC Alternator which will be around 88% Efficient.

Sorry. Too busy to help. I am busy reinventing the wheel. I am looking for a more efficient shape.

"Is this enough information to calculate / validate a suitable Diesel Engine kW with suitable Oversize AC Alternator ( Generator ) to match."
That has already been done by multiple manufacturers of diesel gen-sets.

Go to the Caterpillar website and use their sizing software for a gen-set to meet your needs.
In regards to set capacity: Standby Power = Prime Power + 10%.
But, comparing DOL motor start sizing, there may not be a 10% correlation between a Prime set and a Standby set. (It has to do with motor starting allowable voltage dip.)
Commercial sets are rated at 0.8 PF. kW = KVA x 0.8.
PS: Don't try to reinvent the wheel or gen-sets.

 

[EdStainless]

We commonly drove pumps with engines, usually through gear boxes.
Just make sure that the gear ration is odd (we would use 2.379 or something like that).

 

[waross]

Just make sure that the gear ration is odd

That comment brings back memories.
Case one:
Pressure pulses as pump impeller vanes pass the discharge port.
In the days before "Forbidden Frequencies were widely understood.
Case, A pump driven by a 60 HP motor on a PID controlled VFD.
The discharge straight into a pipe with a relatively long distance to the first bend.
The PID control inadvertently set a forbidden frequency.
Pressure peaks reflected from the downstream fitting reinforced the pressure peaks caused by the impeller blades passing the discharge port.
The resulting pressure piling blew out the side of the cast pump housing.
The hole was about 3 inches by 4 inches, and the material almost 1/2 inch thick.
This illustrated pressure peaks developed by the vanes of a centrifugal pump.

Case two: A new facility (A world scale open pit mine and concentrator) running on diesel generators pending the construction of a 140 kV transmission line.)
The bearings of a new diesel generator failed in a few weeks of operation.
The bearings of a new replacement diesel generator failed in a few weeks of operation.
The cause was determined to be the action of the electric heating controller in the main office building.
The PID control was using a time proportional algorithm with SCR switching.
That is a number of cycles off followed by a number of cycles on.
The timing of the switching cycle coincided with a multiple of the engine speed so that the block loading of the entire heating load hit the crank repeatedly at the angular position.
The heating was transferred to conventional thermostatic control until the power line was completed.
This illustrated repeated block loading at the same rotational position on a crankshaft.

Question?
Was it one or both of these conditions that prompted your advice to use an odd drive ratio, Ed?

 

[EdStainless]

The first. Our pumps were designed for 3600rpm (well 3500).
If you tried to drive them with a 2x gear box, the vibrations would be horrific.
And if you got the speed wrong and vane pass was a multiple of the speed you could watch it self-destruct.
These were multistage centrifugal pumps typically used downhole.
We would mount them in cradles and motor drive them for surface use.
Typically, 5"-8" diameter x 15-20' long, 25-40 stages, and head of 5-8,000 ft.

 

[waross]

Thank you for your information Ed.

The only load is the pump?

I was once faced with that choice.
A large sawmill was contemplating an emergency pump for fire protection.
I strongly advised against a direct motor driven pump and lobbied for a electric pump supplied by a standby generator which would also supply the office load.
Why?
In that culture, (third world) If a pump engine was out of service it may never be repaired until after a major fire, and possibly not even then.
If a generator failed and the office power went out, there would be a much greater possibility of sooner than later repairs.
Just saying.
One size does not always fit all.

 

[edison123]

Why would you size a 800 KVA DG for a 400 KW VFD driven motor?

Motor voltage is irrelevant. Diesels can run generators of any voltage.

 

[waross]

400 kW rating will be a 500 KVA rating.
Add 10% for prime rating. Now 550 KVA
Add 25% for motor acceleration and overloads. Now 687.5 KVA.
Another 112 KVA for harmonic support. Now 800 KVA
This may not be as bad as it seems particularly after 15,000 or 20,000 running hours. (20 months 24/7 at 15,000 hours)

( typically the AC Alternator is connected in Three Phase Zig Zag Cofiguration ).

Never seen that before.
What a waste of almost 20% of capacity.
There is a zig-zag connection used to convert from three phase to true single phase.
There is also a double delta connection used to convert from three phase to true single phase.
Both connections waste 33% of capacity. (Half of the output current is at 50% PF) or (Rated current times two hot lines versus rated current times three hot lines)
The performance is equal.
The double delta looks good and is symmetrical on the office desk.
In the real world, the three phase to zig-zag may be accomplished safely in the field with only a simple continuity tester, even if all the wire markers have been lost or obliterated.

Comparing 415 Volts to 690 Volts for efficient capacity usage.
The generator windings will probably be capable of 280 Volts/60 Hz and 234 Volts/50 Hz. (The ultimate limit as opposed to the nameplate numbers.)
At 415 Volts, the connection will be a series delta with the AVR set to 208 Volts.
At 690 Volts the connection will be a series star with the AVR set to 200 Volts.
The current rating of each winding will be the same regardless of the voltage.
So, the set will have 4% more capacity at 415 Volts.
Actually, 400 Nominal Volts (398 Actual Volts) is the voltage that matches up with 690 Volts.