Chapter A General rules of electrical installation design

L20
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L - Power factor correction and
harmonic filtering
Installation before P.F. correction
� � � (1)
kVA=kW+kvar b kvarh are billed heavily above the declared
level
b Apparent power kVA is significantly greater
� � �
kVA=kW+kvar
kVA than the kW demand
kVA
kW kvar
b The corresponding excess current causes
losses (kWh) which are billed
b The installation must be over-dimensioned
kW
630 kVA
400 V
cos ϕ = 0.75
workshop
Characteristics of the installation
500 kW cos ϕ = 0.75
b Transformer is overloaded
b The power demand is
S = P = 500 = 665 kVA
cos ϕ 0.75
S = apparent power
b The current flowing into the installation
downstream of the circuit breaker is
I = P = 960 A
3U cos ϕ
b Losses in cables are calculated as a
function of the current squared: 960 2
P=I 2 R
cos ϕ = 0.75
b Reactive energy is supplied through the
transformer and via the installation wiring
b The transformer, circuit breaker, and cables
must be over-dimensioned
8 Example of an installation
before and after power-factor
correction
Schneider Electric - Electrical installation guide 2008
Installation after P.F. correction
630 kVA
400 V
cos ϕ = 0.75
workshop
Fig. K27 : Technical-economic comparison of an installation before and after power-factor correction
(1) The arrows denote vector quantities.
(2) Particularly in the pre-corrected case.
b The consumption of kvarh is
v Eliminated, or
v Reduced, according to the cos ϕ required
b The tariff penalties
v For reactive energy where applicable
v For the entire bill in some cases are
eliminated
b The fixed charge based on kVA demand is
adjusted to be close to the active power kW
demand
Characteristics of the installation
500 kW cos ϕ = 0.928
b Transformer no longer overloaded
b The power-demand is 539 kVA
b There is 14% spare-transformer capacity
available
b The current flowing into the installation
through the circuit breaker is 778 A
250 kvar
b The losses in the cables are
reduced to 7782 = 65% of the former value,
960 2
thereby economizing in kWh consumed
cos ϕ = 0.928
b Reactive energy is supplied by the capacitor
bank
Capacitor bank rating is 250 kvar
in 5 automatically-controlled steps of 50 kvar.
Note: In fact, the cos ϕ of the workshop remains at 0.75 but cos ϕ for all the
installation upstream of the capacitor bank to the transformer LV terminals
is 0.928.
As mentioned in Sub-clause 6.2 the cos ϕ at the HV side of the transformer
will be slightly lower (2) , due to the reactive power losses in the transformer.