PFI for Standby Genset

Here are the answers to your questions:

1. Is it wise to install PFI bank to the load to improve standby diesel genset capacity? No

2. Is there any benefit of improve fuel consumption for the same gent set with PFI installation? No

3. How do you size a capacitor bank to improve a pf from 0.7 to 0.9 @ 500 kva load?

See this thread: http://cr4.globalspec.com/thread/54230 for this calculation(comment # 5)

- MS

Can you please tell what is your rational to say no as your answer in Q1?

It is because of the use of 'wise to install' in the question. For further detail please read Tornado's explanation and see the final comment.

- MS

Why do they switch off the capacitor bank when standby generator comes on?.

POWER FACTOR CORRECTION WITH DG SETS - A CASE STUDY

by: K. Sivakumar, Manager-Training, Switchgear Training Centre, Coonoor.

DG Set Details:

Make: Powerica Limited

Frame: 4AB 400/8 Rating: S1 to IS 4722-1992 Exciter: AC

kVA: 1000 Insulation Class: ST-F; RT-F; Ex.-F Altitude: 1000m

RPM: 1500 Hz.: 50 DE: NU 32; NDE: 6324

Volt: 415 Ph.: 3 Castrol AP3

Amps.: 1392 P.F.: 0.8 DE: 72; NDE: 72

Extn.: 96V; Conn.: Y DE: 2000 hrs; NDE: 2000 hrs.

Extn.: 2.6A Coolant Temp.: 40^oC Phase Sequence e: IS 4728

Rotation: CW viewing at driven end M/c. No.: 9530254-06

P. Code: Make: Kirloskar

Engine Model: KTA 3067-G; 1000kVA; 1180 BHP

The rated power of the above Diesel Engine is 1180HP.

Assuming 95% engine efficiency, this is 1121 HP at the engine output shaft.

Which means that the alternator input shaft power is 1121HP.

The efficiency of 1000kVA, Kirloskar Electric alternator is 95.4%.

Hence, this input shaft power of 1121 HP gets converted to 1069.43HP at the alternator output; this is 1069.43 x 0.746 =797.79kW, say, 800kW.

So, it is established that the above combination of a 1180HP Diesel Engine with a 1000kVA alternator MUST be loaded only upto a maximum KILOWATT loading of 800kW; any increased kW loading would only overload the engine and the engine would stop.

So, there is no point in arguing that by adding capacitors, one can improve the power factor of the alternator supplied loads to say, Unity and thus loading the DG set to (1000 x1.0) 1000kW instead of 800kW.

Let us analyse the case.

Let us assume that one has connected capacitors to the DG set and improved the PF to, say, 0.98; he now loads the engine to 980kW, going by the formula that kW = kVA x PF (1000 x 0.98). Now, the 980 kW load connected to this alternator would be reflected as (980 / 0.954) 1027.25kW at the alternator input shaft and at the engine input this is reflected as (1027.25 / 0.95) 1081.32kW; converted to HP, this is (1081.32 / 0.746) = 1449.49HP; but the engine rating is only 1180 HP; thus, the engine would get overloaded by 123% and would thus stop/trip on 'engine overload'.

But, then should we add capacitors to the DG Set power supply or not?

Definitely, adding capacitors in the network when DG Set is feeding power is very much advantageous.

Suppose the above DG set is loaded to 800kW (without overloading the engine).

Now, if the power factor is 0.8, the current thro the alternator windings would be: 1391.21A.

If the power factor is improved to, say, 0.98, the current then would be: 1135.71A only.

There will be a reduction in current of 255.5A;

The alternator efficiency is 95.4%; that means for delivering 800kW output load, the input power requirement would be 838.57kW;

Ignoring iron losses, we can assume that the I²R losses in the alternator is 38.57kW (for a current of 1391.21A). Now, if the current is 1135.71A, the I²R losses in the alternator would only be:

{[(38.57) / (1391.21)²] x (1135.71)²} = 25.70kW.

There would be a saving of 12.87kW in the I²R losses in the alternator.

That means that the engine needs to supply that much less kW now. The full load fuel consumption of the above engine is 209 LPH; that means with 209 LPH, it can generate 800kWh; thus, the specific fuel consumption is 3.83 Units per Liter of Diesel.

With 12.87kW saving in power, the saving in fuel will be (12.87 / 3.83) 3.36 Litres per hour. That is Rs. 150.00 savings per running hour of the DG Set, assuming a diesel price of Rs. 44.00 per Liter.

This, in fact, is the advantage of adding capacitors, even when the DG Set is supplying power.

But, a word of caution though:

The power factor compensation system MUST be an Automatic Power Factor Correction (APFC) System and NOT a manual one. Because, with a manual PF correction system and with varying loads on the DG set, it may so happen that the power factor might go leading. Leading power factor is very dangerous to the alternator and to its sub-systems such as AVR, the Exciter wheel diodes, the surge suppressors in the excitation system and even to the alternator stator windings and to the rotor windings. Because any alternator is meant only to "supply" reactive power and not to "absorb" reactive power.

I remember for large induction motors which draw high starting current at low pf,alternator which is bigger than the diesel engine was recommended and for some other applications engine larger than alternator was recommended. For sizing standby/emergency generators larger set was recommended if it incorporates turbo charger as the full load is applied soon after starting.

You are entering a new field of discussion altogether. This is called Transient kVA requirement of the Generator. This has no effect whatsoever on the subject discussion of pf control of the loads on the alternator.