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Friday, March 20, 2009

KW, KVA, Kvar, Explained




The explaination of the difference between KW, KVA and KVAR are always difficult concepts to get across, but these values are very important to understand as designers. upwards of 90% of all the worlds energy is used by motors, think about your own home, in the kitchen you will have fans in the oven, motors in the microwave, motors in the dishwasher, extract fans etc, they are all around us, and it is only by understanding the influance of load and power can we understand how to best maximise the effeciency not only of a single unit of equipment but an entire building as a whole. It all comes down to your power factor, if your building has a large number of inducive loads e.g. motors then you may require PF correction, some utilities companies will charge additional for a meter point having a poor PF.



Apart from a small variety of electrical equipment such as heating apparatus, the majority of
domestic and industrial equipment require reactive power to operate. Capacitors can be
used to provide the reactive power that electrical devices need.
Capacitors can be installed on the network called a capacitor bank at the clients side of the meter point or at the device requiring the reactive power.



Power factor is a measure of the relationship between the real and reactive power taken by
your electricity installation. Your electricity power supply is an alternating current (AC) electrical system. AC systems have two relevant components, real power and reactive power.
Real power is the power required to do the actual work, for example, the power that produces the actual heat, and is measured in kilowatts (kW).
Reactive power is the power required to energise electric and magnetic fields that are ancillary to the production of real power. Reactive power is measured in kilovolt Amps
reactive (kVAr).


In terms of power, real power may be considered the horizontal force because it does
the actual work. Most electrical apparatus (loads) in a plant also require a magnetic field to
operate. Reactive power provides this field, although it does not do any useful work.
These two components exist together in all AC systems.


Power factor is important because transmission and distribution systems must be designed and
built to manage the need for real power as well as its associated reactive component – the total
power. The total power can be greater than 50 percent more than real power alone if the power
factor is poor.
Ideally, power factor should be as close to unity
as possible, Unity is = to 1 if a system adds more capacitance to there incoming power and if not configured correctly it could cause a leading power factor. The further the power factor moves
away from unity, the more network capacity is needed to supply the same amount of usable
power.


Power calculations.

P = Power electrical

V = Voltage

I = Current

PF = Power Factor.

Square root of 3


If we have a new piece of plant being installed, three phase 400v, 50Hz (UK system) the frequency and voltage is different in the US etc. we know the power of the plant and the voltage and the current it will draw. to calculate the PF we do as follows. the measured value of I = 20A, P = power 10 Kw load.



PF = P/(VxIxsq root of 3)


PF = 10,000w / 400 x 20 x sq root 3


this gives us a PF of 0.72 this is quite low PF and it would be advisable to add capacitance to bring it closer to unity (1).


Now we know the PF we can calculate the KVA.


Kva = KW/PF


10 kw/0.72pf = 13.88 KVA.


This is the apparent power value.


we can also get the KVAr which is the reactive power value as follows.


we get the inverse sin and then cos of the PF.


which gives us a value of 0.69, we take this figure.


0.69 x KVA = Kvar


0.69 x 13.88 = gives us a value of 9.57 Kvar.

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