Priemysel Kompenzacia Aples Technologies Katalog-F

Operation, protection and connection of
capacitors
PROTECTION AND CONNECTION OF CAPACITORS

GENERAL INFORMATION
Operating device Protection

In the case of loads with ultra-fast cycles (welding In addition to the internal protection devices
machines, etc.), the conventional system for operating incorporated in the capacitor:
capacitors (electromechanical contactors) is no longer - Self-healing metallised film
suitable. High-speed switching compensation systems - Internal fuses
using solid state contactors are necessary. - Overpressure disconnection devices
LEGRAND offers this type of equipment. it is essential to provide a protection device external to
the capacitor.
The switching current of a capacitor depends on:
This protection will be provided by:
The power of the capacitor
The short-circuit power of the mains supply to which Either a circuit breaker:
it is connected - Thermal relay, setting between 1.3 and 1.5 In
Whether or not any capacitor banks that have already - Magnetic relay, setting between 5 and 10 In
been activated are present
Or GI type HRC fuses, rating 1.4 to 2 In
Given these parameters, it is essential to use quick
make and break operating devices (switch, contactor, In = capacitor nominal voltage
etc.).
When selecting operating devices, the user must be In = Qc/ 3U
made aware of the choice of equipment available (for Example: 50 kVAr - 400 V three-phase
operating capacitors).
Contactors are specially designed by contactor In = 50/1.732 x 0.4 = 72 A
manufacturers for operating capacitors and in
particular for assembling automatically controlled
capacitor banks. These contactors have auxiliary poles Connection (sizing the cables)
combined in series with preload resistors that will limit Current standards for capacitors are defined so that
the inrush current during activation. capacitors can withstand a permanent overcurrent of
30%.
These standards also permit a maximum tolerance of
10% on the nominal capacitance.
Cables must therefore the sized at least for:
I cable = 1.3 x 1.1 (I nominal capacitor)
i.e. I cable = 1.43 I nominal
16
Harmonics
INTRODUCTION
GENERAL INFORMATION
In recent years, the modernisation of industrial The FOURIER decomposition (harmonic analysis) of the
processes and the sophistication of electrical machines current consumption of a non-linear receiver shows:
and equipment have led to major developments in The fundamental, a sinusoidal term at the 50 Hz
power electronics: mains supply frequency
The harmonics, sinusoidal terms whose frequencies
Semi-conductor-based systems (transistors, thyris- are multiples of the fundamental frequency
tors, etc.) designed for:
Static power converters: AC/DC According to the equation:
Rectifiers
n
Inverters Irms = I21 + h∑= 2 I2h
Frequency converters
And many other multicycle or phase controlled ∑ : Sum of all the harmonic currents from harmonic 2 (50 Hz x 2) to the last
harmonic n (50 Hz x n)
devices.
These systems represent "non-linear" loads for
electrical supplies. A "non-linear" load is a load for These harmonic currents circulate in the source. The
which the current consumption does not reflect the harmonic impedances of the source then give rise to
supply voltage (although the voltage of the source harmonic voltages, according to the equation:
imposed on the load is sinusoidal, the current Uh = Zh x Ih
consumption is not sinusoidal).
Other "non-linear" loads are also present in electrical The harmonic currents give rise to most of the
installations, in particular: harmonic voltages causing the overall harmonic
Variable impedance loads, using electric arcs: arc distortion of the supply voltage.
furnaces, welding units, fluorescent tubes, discharge
n
lamps, etc. Vrms = U21 +h∑= 2 U 2h
Loads using strong magnetising currents: Note: The harmonic distortion of the voltage generated by construction
saturated transformers, inductors, etc. defects in the windings of the alternators and transformers is generally
negligible
17
Harmonics (continued)
EFFECT OF HARMONICS ON CAPACITORS

GENERAL INFORMATION
Sous-Titre
Schematic 1
diagram Equivalent diagram Main harmonic currents
Texte courant Texte courant Texte courant Texte
courant Texte courant Texte courant Texte courant
XLT : SC C (kVA) The main harmonic currents present in electrical
Texte courant Texte courant Texte courant Texte installations come from semi-conductor based
courant Texte courant Texte courant Texte courant systems. The theoretical rates of such systems are as
Texte courantM Texte courant Texte XLT courant
XC Texte
R follows:
XC ±
courant
Q(kvar) Texte courant Texte courant Texte courant - Harmonic 5 (250 Hz) - I5 - 20% I1*
Texte courant Texte courant Texte courant Texte - Harmonic 7 (350 Hz) - I7 - 14% I1*
courant Texte courant Texte courant Texte courant - Harmonic 11 (550 Hz) - I11 - 9% I1*
R L
Texte courant - Harmonic 13 (650 Hz) - I13 - 8% I1 *
P (kW)
Note: As the inductance of the motor is much higher than that of the source, (* I1: Semi-conductor system current at 50 Hz)
it becomes negligible in parallel configuration.
Ssc (kVA): Source short-circuit power

Q (kVAr): Capacitor bank power
Parallel resonance or anti-resonance
P (kW): Non-interfering load power between capacitors and source
XL
The reactance of the source XLT
XLT is proportional to the frequency
Reduction of the reactance of the The reactance of the
capacitors XC is inversely
capacitors Fr.p. f (Hz)
proportional to the frequency
XC
XL The capacitor reactance At frequency Fr.p., there

is parallel resonance or
1 1 XC anti-resonance (as the two
XC = =
f (Hz) C. t C.2./.f reactances are equal but
opposite ) and amplification
XC is inversely proportional to
(F.A.) of the harmonic currents
the frequency, its ability to
in the capacitors and in the
cancel out harmonic currents
source (transformers) where:
XC decreases significantly when
the frequency increases.
Ssc Ssc. Q
Fr.p. = F supply F.A. =
Q P
s4HEHIGHERTHESOURCESHORT CIRCUITPOWER3SC
the further the resonance frequency is from
dangerous harmonic frequencies.
s4HEHIGHERTHEPOWER0 OFTHENON POLLUTING
loads, the lower the harmonic current amplification
factor.
18
PROTECTING CAPACITORS USING DETUNED REACTORS
GENERAL INFORMATION
For supplies with a high level of harmonic pollution, The most commonly used F.r.s values are:
installing an detuned reactor, tuned in series with the - 50 Hz fundamental: 215 Hz (n=4.3)
capacitor, is the only effective protection. 190 Hz (n=3.8)
135 Hz (n=2.7)
The detuned reactor performs a dual role:
Increasing the impedance of the capacitor in relation - 60 Hz fundamental: 258 Hz (n=4.3)
to the harmonic currents 228 Hz (n=3.8)
Shifting the parallel resonance frequency (Fr.p) of the 162 Hz (n=2.7)
source and the capacitor to below the main frequencies
of the harmonic currents that are causing interference For frequencies below Fr.s., the reactor/capacitor
system behaves like a capacitance and compensates
Fr.p.: Detuned reactor/capacitor/MV/LV transformer the reactive energy.
parallel resonance frequency For frequencies above Fr.s., the reactor/capacitor
system behaves like an inductance which, in parallel
Fr.s.: Detuned reactor/capacitor serial resonance with the inductance XLT, prevents any risk of parallel
frequency resonance at frequencies above Fr.s. and in particular
at the main harmonic frequencies.
HARMONIC FILTERS
For installations subject to a high level of harmonic For this type of installation, LEGRAND offers the
pollution, the user may be faced with a dual following services:
requirement: Analysis of the mains supply on which the equipment
To compensate for the reactive energy and protect is to be installed, with measurement of harmonic
the capacitors voltages and currents
To reduce the harmonic distortion of the voltage to Computer simulation of the compatibility of the harmonic
values that are acceptable and compatible with correct impedances of the supply and the various filters
operation of most sensitive receivers Calculation and definition of the various components
(PLCs, industrial computers, capacitors, etc.) of the filter
Supply of capacitors, inductances, etc.
For this, LEGRAND can offer "passive" harmonic Measurement of the efficiency of the system after
filters. A "passive" harmonic filter is a combination of installation on site
a capacitor and an inductance in series, for which each
tuning frequency corresponds to the frequency of an
unwanted harmonic voltage to be eliminated.
19

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Operation, protection and connection of

PROTECTION AND CONNECTION OF CAPACITORS

Operating device Protection

i.e. I cable = 1.43 I nominal

EFFECT OF HARMONICS ON CAPACITORS

Ssc (kVA): Source short-circuit power

XL The capacitor reactance At frequency Fr.p., there

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Priemysel Kompenzacia Aples Technologies Katalog-F

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Priemysel Kompenzacia Aples Technologies Katalog-F

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Copyright

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Priemysel Kompenzacia Aples Technologies Katalog-F

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Operation, protection and connection of

PROTECTION AND CONNECTION OF CAPACITORS

Operating device Protection

i.e. I cable = 1.43 I nominal

EFFECT OF HARMONICS ON CAPACITORS

Ssc (kVA): Source short-circuit power

XL The capacitor reactance At frequency Fr.p., there

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Ssc (kVA): Source short-circuit power

XL The capacitor reactance At frequency Fr.p., there