Understanding
Power Factor
Layman’s guide
What is Power Factor?
Power is used most efficiently if the current is aligned with the
voltage. Some light bulbs and heaters draw current exactly
synchronized to voltage, but most other loads tend to draw current
with a delay (lag). It takes more current to deliver a fixed amount of
power when the current is lagged or delayed.
In c r e ase i n Cu r r e n t w i th Po o r Po w e r F ac to r
Beer (kW) 80%, Froth (kvar) 20%, Your
pint (kVA) 100%.
• Why pay for froth?
• Reducing the percentage of froth
allows you to have almost a full pint.
• You can’t get rid of all the froth – but
a little is ok!
220%
Reactive
18 0 %
Power
16 0 %
(kvar)
140 %
12 0 %
10 0 %
1
0 .9
0 .8
0 .7
0 .6
0 .5
Pow er Factor CosØ
The ratio of the actual power transmitted “real power” (kW) to the
“apparent power” (kVA) that could have been transmitted if the
same current were in phase and undistorted is known as the power
factor. The delay or phase shift is caused by a requirement for
active power (kW) and reactive power (kvar). Reactive power is
required by motors, transformers etc to magnetise the circuit. This
reactive power can either be supplied by the electricity supplier in
the form of penalties or by capacitors connected to the system.
Technical Notes
KW = kVA x p.f.
KW = 354 x 0.79
= 279.6kW
Kvar = kVA2 - kW2
= 125316-78176
= 217.1kvar
adding 150kvar makes
site kvar 67.1.
KVA = kW2 + kvar2
= 78176 + 4502
= 287kVA
Saving 67kVA or 19%
Apparent Power (kVA)
Current
200%
R eal
Power
(kW)
Example
A site in the South East of England purchases their electricity from Southern
Electric, but is connected to the local EDF network.
The site has a Maximum Demand (M.D.) of 354kVA and a Power Factor of 0.79.
Installing a 150kvar Low Voltage PFC bank increased the power factor to 0.97,
saved the company 67kVA, and 21,711kvarh units per month.
Based on their current tariff they realised £1,615.30 p.a. total savings:
Reactive Power charge
(charged at 0.62p per kvarh excess)
Saving 21,711kvarh x 0.0062 x 12 months
= £1,615.00 p.a.
Cost of equipment including installation
Totalled £1,900
Payback Time = 14 months
What type of Power Factor Correction?
100%
Power factor correction equipment is available in various
forms and is very easily misapplied. Choosing the right
type of correction is essential in getting good long-term
benefit for the equipment.
Standard or De-tuned?
The choice of equipment is dependent upon the nature of
the loads connected. If there are any non-linear loads
connected these may adversely affect the equipment:
Absorb harmonic currents, leading to premature
failure
Make the power quality for connected equipment
worse.
While standard PFC is approximately half the price of the
equivalent de-tuned type if installed in site with a
considerable amount of non-linear load its operating life
is cut to months.
Special PFC/
Filter
Percentage of non-linear
loads such as Computers,
Variable Speed Drives,
UPS etc.
50%
De-tuned
PFC
15%
Standard
PFC
None
General rules of thumb; if total non-linear load
percentage exceeds 15%, detuned PFC must be
used. Exceeding 50% requires a special filter.
Low Voltage PFC
Correction of Power factor at low voltage is the
usual convention. Banks are low cost and come in
a range of different types to fit the customers
individual needs.
Medium Voltage PFC
An example of a Low Voltage (400V) PFC bank
In large facilities Medium Voltage compensation
may become a lower cost option. This enables
one large bank to provide the compensation for a
number of supplies.
The PQM Difference
Involved in electrical engineering since 1976, there are
not many technical problems that we haven't seen. Our
broad experience includes work in energy measurement,
product design, installation and maintenance. This gives
us a strong appreciation of our customers' needs and
expectations, and also the means to meet their
requirements.
In recent years we have focussed on identifying,
quantifying and correcting 'power quality' and 'power
factor' problems; having successfully assessed and applied
solutions to over 550 installations - both at Low and
Medium Voltage.
An example of a Medium Voltage (11kV) PFC bank
Contact us:
Power Quality Management Ltd
PO BOX 76
Blakeney, GL15 4WU
Phone: 01594 510670
Fax: 01594 516170
E-mail: info@pqmanagement.co.uk
www.pqmanagement.co.uk