Download rtfx harmonic problems in capacitor banks

QUALITY AND TECHNOLOGY
HARMONIC
PROBLEMS IN
CAPACITOR BANKS
quality of the
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Common problems:
Capacitor banks provide power
correction to correct low cos-phi.
factor
Nowadays more and more harmonic
generating loads are used (frequency
converters, UPS, soft starters, etc.) causing
elevated levels of harmonics.
When used in electrical installations where harmonics are present, the
application of a conventional capacitor bank is likely to suffer and cause
several problems:
1) Failure and short lifetime of power capacitors
2) Elevated voltage harmonics
3) Danger of resonance between capacitor bank and main transformer
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Problem 1: Failure of capacitors
Current through
the capacitor
The current through the capacitor depends on
the impedance of the capacitor.
Icap = V / Zcap
The impedance of the capacitor is much
lower for higher frequencies (harmonics).
Impedance
Zcap
Zcap = 1 /(2 f C)
Z1
Therefore harmonics in the voltage cause
elevated additional harmonic currents
through the capacitor which overload and
damage the capacitor
Example:
3% of 5th harmonic in voltage
causes 15% additional current
through capacitor
Z3
Z5
fundamental
3rd harmonic
5th harmonic
Frequency
f
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Problem 2: Elevated voltage harmonics
RESONANCE
L
DISTRIBUTION
TRANSFORMER
C
HARMONICS OF THE
SAME FREQUENCY
CAPACITOR BANK
Example:
30% of harmonics in the current, will normally cause
around 3% harmonics in the voltage. However a capacitor
bank may increase the voltage harmonics to 8% or more.
Tip:
To know the effect of the capacitor bank on the
harmonics, compare measurements of harmonics with and
without connected capacitor bank.
The capacitance ‘C’ of the
capacitor bank and the
inductance ‘L’ of the main
transformer and cables form an
L-C circuit with certain
resonance frequency.
In
case
the
resonance
frequency is close to one of the
present
harmonics,
this
harmonic will be amplified,
especially in the voltage.
Harmonics cause overheating
and malfunction of equipment.
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Problem 3: Resonance
DISTRIBUTION
TRANSFORMER
L
DESTRUCTIVE
RESONANCE
C
HARMONICS OF THE
SAME FREQUENCY
CAPACITOR BANK
Note:
Resonance is a serious risk which shall not be under
estimated, especially in installations with low
impedance.
In
case
the
resonance
frequency of the LC circuit
formed by the capacitor bank
and the main transformer
coincide with a present
harmonic in the installation, a
complete resonance can occur.
The resonance may have
destructive results for the
capacitor
bank,
main
transformer
and
main
switchboard.
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Solution: Filtering inductors RTFX
RTLX filtering
L
The filtering inductor provides a high
impedance
for
high
frequencies
(harmonics). The total impedance of the
capacitor + inductor step is inductive
above the so-called ‘resonance frequency’
inductor
Total current
C
Impedance
|Z|
I = V / Ztotal L + C
Z = 2 f L + 1 /(2 f C)
Benefits:
Capacitive
impedance
Z1
Inductive
impedance
Z5
Z3
Resonance
frequency
fundamental
3rd harmonic
-High impedance for harmonic
frequencies:
avoids
high
harmonic currents through the
capacitor
-Inductive impedance above
resonance frequency: avoids
Frequency amplification of harmonics and
f
5th harmonic
risk of resonance
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Technical considerations 1
RTLX filtering
L
inductor
Total current
C
Impedance
|Z|
I = V / Ztotal L + C
Z = 2 f L + 1 /(2 f C)
Capacitive
impedance
Z1
Z5
Z3
fundamental
3rd harmonic
Inductive
impedance
The resonance frequency is normally
chosen at 189Hz (p=7%), however, in case
significant 3rd harmonics are present, it is
recommended to choose 135Hz (p=14%)
The
resonance
frequency
depend on the capacitor C and
the inductor L, it is very
important that both components
are well tuned to each other.
A wrong resonance frequency
may
result
in
harmonic
Resonance
absorption causing damage.
frequency
Both
capacitance
and
Frequency inductance
values
must
f
5th harmonic
therefore be correct and stable
to guarantee correct tuning
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Technical considerations 2
Over voltage:
L
C
189Hz: 7,5%
135Hz: 14%
Filtering inductors cause a higher voltage on
the capacitor terminals. The capacitors should
therefore be suitable for this higher voltage.
Examples, for a 400V, 50Hz network:
Inductors tuned to 189Hz (p=7%):
Use capacitors for 440V, 460V, 480V, 500V or 525V
Inductors tuned to 135Hz (p=14%):
Use capacitors for 480V, 500V or 525V
The effective kvar rating of the capacitor + inductor is
calculated as follows:
189Hz: P_eff = (V_line/V_cap)2 x 1,075 x P_cap
135Hz: P_eff = (V_line/V_cap)2 x 1,14 x P_cap
Example: line: 400V, 50Hz, capacitor: 440V, 50kvar
inductor tuned to 189Hz:
P_eff= (400/440)2 x 1,075 x 50 = 44,4 kvar
Note:
Filtering
inductors
are
recommended for electrical
installations where the level of
voltage harmonics THD%(V) is
2,5%..3% or more
HARMONIC PROBLEMS IN
CAPACITOR BANKS
RTFX
FILTERING INDUCTOR FOR
CAPACITOR BANKS
ADVANTAGES:
- Copper Windings
- Problem free terminals
- Class F insulation
- Low temperature rise (< 90ºC)
- Competitive prices
HARMONIC PROBLEMS IN
CAPACITOR BANKS
Standard characteristics
Line voltage
400V, 50Hz
Capacitor voltage
440V, 50Hz
Resonance frequency
189Hz
Isolations
Class F
Windings
Copper, class HC-200ºC
Terminals
Copper cable terminals
Test voltage
3 kV (1 min, 50 Hz)
Harmonic currents
I3=8%, I5=31%, I7=13%
Thermal overload factor
5%
Over current
6%
Tolerance
3%
Protection degree
IP-00
Ambient temperature
40ºC
Temperature rise
<90ºC
Standards
IEC/EN/UNE-EN 60289, CE
Includes
Bimetal temperature sensor, 120ºC, nc
Other characteristics on request
RTFX
POLYLUX, S.L.
C/ Boters 3b
Parc Tecnològic del Vallès
08290, Cerdanyola del Vallès
(Barcelona) Spain
tel.: +34-936926565
fax: +34-935809603
[email protected]
www.polylux.com
(please indicate line voltage / frequency and capacitor rating / voltage / frequency)
HARMONIC PROBLEMS IN
CAPACITOR BANKS
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