engine generator set power rating

[Mr zezo]

engine generator set power rating nameplate is
KVA,KW and PF what does this mean ?
what i excepect is that the generator power is rated as KVA only and i can load it with any power factor according to load type ,
but at any nameplate KVA,KW and PF are given ,

the misunderstading here is the difference between engine generator set given power factor and electrical load capacity i can use regarding power factor?
when sizing a generator required for any building or factory ; if i get sum of all loads KVA plus adding saftey margin ,then i should select the generator according to the calculated loads kva whatever the power factor is ?
what about this conception ,when take engine power into consideration,let engine 100 hp which equals to 100×.746=75 KW; which means the load of the generator must not exceed 75 KW .then i can said that the engine generator set power is 75 KVA what ever the load power factor.!

 

[Gr8blu]

KVA - apparent power (a combination of real usable power and reactive power).
KW - real power (the stuff you're actually using to perform useful work)
KVAR - reactive power (the stuff you can't use to perform useful work)
PF - power factor, which is the ratio of real power to apparent power

KVA = [(KW)^2 + (KVAR)^2]^0.5
KW = KVA * PF

Reactive power is a result of changing the excitation (amount of energy) as an input to the rotor winding. More excitation means export of reactive power; less excitation means import of reactive power. Thus the ability of the rotor to eliminate thermal stress (from over-excitation) is a practical limit on how much reactive power can be exported.

All generators have a "capability curve". This curve is defined by the amount of real and reactive power available at any given time - with further limits given by rotor heating (i.e. the maximum PF in the over-excited condition), unstable operation (i.e. there is insufficient excitation to keep the generator synchronized in the event of an abrupt load step), and the stator winding heating (i.e. the point where KVA = KW and PF = 1.0).

The idea is that the plant (facility) loading has a better (i.e. closer to 1.00) power factor than the generator rating. Typical generators have fairly standard PF ratings (1.0, 0.9, and 0.8 are the most common). More reactive power is necessary when starting large loads such as motors due to the high inrush current and poor power factor of the loads during the transient, even though the transient is of fairly short duration. It is also usually a good idea to have a bit of excess real power (KW) compared to the plant load to account for load increases through the design life of the facility.

The engine KW output has to be enough for the desired generator output KW plus system losses. Typically the driver is sized for at least (output + 10%).

Converting energy to motion for more than half a century

 

[waross]

The capacity of the generator end is determined by heat which is related to the current.
The KVA rating is the maximum output that the generator may develop.
But, the main limit is current and heating.
As well as KVA you must consider the maximum rated amperes.
eg. a 3 phase generator is rated for 41.6 KVA at 240 Volts.
The maximum rated current is 100 Amps.
The generator is to be used at 208 Volts.
208 Volts 3Ph at 100 Amps = 36 KVA.
so the maximum safe load that the 41.6 KVA, 240 Volt generator may carry at 208 Volts is 36 KVA
Industrial loads are generally at less than unity power factor.
The kW rating is determined by the maximum power that the prime mover is capable of delivering.
It is an industry standard to size the prime mover kW at 80% of the generator KVA rating.
This applies to standby rated sets.
Prime power sets are de-rated 10% for both kW and KVA.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Engine capacity decides the kW rating of the Engine-Generator set. The current rating of the Generator and the Excitation system capability decide the kVA of the Generator.
When an Engine-Generator has to start a motor on DoL, the Generator kVA is selected to be higher than normal.
Also, in case of captive power generation in an industry, the Generator kVA is selected to be larger than normal so that the Maximum Demand of the plant can be met using the Generator and Penalties from the Power company are avoided.
Higher the kVA of the Generator, lower will be its Power factor rating.

R Raghunath

 

[waross]

Higher the kVA of the Generator, lower will be its Power factor rating.

Small sets, below about 15 KVA or 20 KVA may be rated at 100% PF.
Above that I have serviced and installed sets up to 1.6 Mega watts.
Up to that level, a PF rating of 80% is standard.
The kW is the MINIMUM power rating of the prime mover.
I have seen sets spec'ed with the prime mover oversized by 25%.
This is based on the premise that the set may remain in use for 20 or 39 years without an overhaul, and needs to have the power to support the load even when it is worn out and in need of a major overhaul.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

I have seen Diesel Generator sets rated at 0.7 power factor. These were part of captive generation in industry.

R Raghunath

 

[waross]

Fair enough.
I have seen off standard ratings on special military specs.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[charls1]

Hi zezo (Electrical),

- KVA (Kilovolt-Ampere) means the apparent power in a circuit, which is the combination of both real power (KW) and reactive power. KVA is the vector sum of KW and KVAR (reactive power) and is measured in kilovolt-amperes.
- KW (Kilowatt)means represents the real power in a circuit, which is the power actually consumed by the electrical equipment to perform work. KW is the portion of the apparent power (KVA) that performs useful work and is measured in kilowatts.
- PF (Power Factor)means a dimensionless number between 0 and 1 that represents the ratio of real power (KW) to apparent power (KVA) in a circuit. Power factor indicates how effectively electrical power is being converted into useful work output. A high power factor (close to 1) indicates efficient utilization of electrical power.

KVA is the total power, KW is the actual working power, and PF is the efficiency factor that shows how effectively power is being used.