ELECTRONICON KONDENSATOREN GMBH
GERA Ÿ
GERMANY
Introduction
The following technical note provide planners and users with an overview of
fundamentals and practice, as well as a description of our products and
services related to reactive power compensation and power quality - two topics
that are closely linked; because problem optimised reactive power
compensation achieves an improvement of mains and power network quality.
We have attempted to make your choice of the correct solution for your problem
as simple as possible. The hurried, well versed expert will find the information
he needs quickly with the help of the list of contents or the problem oriented
index selection scheme. Those who require more comprehensive information
should take the time to read on.
Many electrical devices, e.g. single-phase and three-phase motors, require
inductive reactive power Q for the build-up and decay of their magnetic fields, in
addition to the active power that is transformed into mechanical work. These
inductive loads draw a current I from the power network that lags behind the
voltage U.
Voltage and current at an inductive load
voltage U
current I
0
90
180
270
360
450
540
630
We have taken care to include in this folder all that you need from the planning
phase to placing your order. This does not however release you from
conscientiously clarifying each case, taking local conditions and the
applicable regulations into account. If you are in doubt, we will gladly
provide you with advice and practical support and, if you wish, even during
your preliminary planning.
Basics of reactive power compensation
Electricity is the most important and cleanest form of energy and its
application is virtually unlimited due to its convenient transmission. The
transmission of active energy is uppermost. It is released by electrical
consumers, e.g. as mechanical work, heat or light.
Consumers that draw only active power P from the power system are referred to
as resistive loads. These include filament lamps and electrical heaters. The
current I drawn by them from the power network has no phase displacement
relative to the network voltage U.
Voltage and current at a resistive load
voltage U
current I
All transmission devices must also be designed to accommodate the additional
reactive current component - this costs a lot of money.
Reactive work is normally measured by the energy supplier and billed to
commercial users due to the additional losses and loading.
reactive power Q
Chapter 1
In contrast to active energy, reactive energy is not drawn continuously from the
power network, but swings between the load and the power station generator. It
loads the transmission equipment and leads to undesirable additional losses
and thus to considerable pollution by CO².
active power P
In this note MKP capacitors means Metalliziert Kunstoff Polypropylene which is
the same as MPP Metallised Polypropylene, the term popularly used in India.
M
This can constitute a significant part of the electricity bill.
0
90
180
270
360
450
540
630
The useless reactive energy component of the transmitted energy should be
kept as small as possible to minimise pollution as well as equipment and
energy costs.
Since reactive power is however indispensable for the consumer, an attempt
must be made to provide it from sources other than the power network.
In contrast to inductive loads, capacitors possess the ability to draw a current
I from the power network that leads the network voltage U. They require
capacitive reactive energy to build up their electric field.
1
Voltage and current at an inductive load
voltage U
current I
0
90
180
270
360
450
540
In the case of an unknown phase angle j
, the apparent power can also be
calculated from the root sum of squares of active and reactive power:
S=
P²+
Q²
630
Sinusoidal voltage and non-sinusoidal current
M
reactive power Q
active power P
An inductive reactive current and an equally large capacitive reactive current
cancel each other. The inductive reactive current, e.g. from motors, can
therefore be compensated by the capacitive reactive current from capacitors.
This process is known as reactive current or reactive power compensation.
Equations and relationships
Sinusoidal voltage and sinusoidal current
The apparent power S for sinusoidal voltage and sinusoidal current is
calculated as the product of voltage U and current I:
The current in most power networks has serious harmonic distortion, i.e.
non-sinusoidal, due to increasing use of equipment with a non-linear currentvoltage characteristic. The voltage normally has a lower distortion, which can
be neglected in power calculations.
The active power can be generated only between voltages and currents of the
same frequency. The harmonic currents can generate only the reactive power
component D with the fundamental component of the voltage. This reactive
power caused by the harmonically distorted current is referred to as harmonic
reactive power.
In contrast to this, the reactive power Q1 caused by the phase displacement j
1
is known as the displacement reactive power. Cosj
1 is therefore also known as
the displacement factor.
The apparent power is therefore:
S=
P²+
Q 1 ²+
D²
Active power is transported only in the fundamental wave:
P=
U×
I1 ×
cosj
1
The ratio between active power and apparent power is known as the power
factor l
:
S = U·I
P
P
l
=
=
S
P²+
Q1 ²+
D²
For any phase anglej
, the active power P is:
The displacement factor cosj
1 results from:
P = U · I · cosj
=
S · cosj
Cosj
is a measure for the ratio of active to apparent power:
cos j
=
The reactive power Q for a sine waveform is calculated from:
Q = S · sinj
P
P
cosj
=
1 =
S1
P²+
Q1 ²
It is also known as the power factor of the fundamental in the special case of
sinusoidal voltage and non-sinusoidal current.
The form of all quantities (power factor, displacement factor, apparent power,
active power, reactive power and harmonic reactive power) can be
comprehended easily with the aid of a diagram with a right-angled coordinate
system.
2
The following relationship applies between the powers:
Individual compensation with automatic capacitor banks can be beneficial for
large loads with widely varying reactive power needs.
S² = P² + Q² + D²
Group power factor correction
In the case of group compensation, several, mostly neighbouring,
simultaneously operating inductive loads e.g. in one sub-distribution, are
grouped together and compensated there by one fixed capacitor. The load group
These comments should give a feeling for the relationships. A deeper
consideration of the complex topic of harmonic reactive power would go beyond
the aims of this document and is unnecessary for its terms of reference.
Chapter 2
Types of compensation
Individual power factor compensation
Individual power factor compensation is typical for the operation of individual
asynchronous motors, generators, transformers, welding devices, discharge
lamps and as a harmonic absorber at larger harmonic sources. Individual
compensation compensates inductive reactive power immediately at the place
of origin. A corresponding capacitor is allocated to each inductive load.
Benefits of individual compensation:
can consist, for example, of motors or lighting with discharge lamps.
If the load and the capacitor are connected jointly to the power network
by means of one contractor or switch, then separate switches are also
unnecessary for group compensation.
If the loads are switched separately, then the capacitor must be
equipped with a separate switch device, which is closed only when all
loads are in operation.
In contrast to individual compensation, the load on the feeder lines to
the individual loads is not removed, but only the feeder to the load group.
Larger lighting systems are often compensated in groups for economic
reasons.
Benefits of group compensation:
Fewer separate components than with individual compensation
l
Unloading of the feeder to the group distribution
l
Central compensation with automatic capacitor banks
Consumer installations usually possess a varying reactive power
requirement. Unregulated fixed compensation is not possible in such
cases, since an uneconomical under-compensation or a dangerous overcompensation can occur. One must therefore continuously adjust the
capacitor power to the required reactive power requirement.
Reduction of burden on the feeder to the load
l
Additional switchgear normally unnecessary
l
Individual compensation is economical for loads with:
Higher powers (> 20 kW)
l
One achieves this by means of automatic capacitor banks, which are
directly assigned to a switchgear unit, distribution, and
sub-distribution or to a large load with fluctuating reactive power
requirements.
Constant power
l
Mainly continuous operation
l
3
Apart from the power section with fuses, switches, reactors and capacitors,
automatic capacitor banks contain a reactive power regulator, which measures
the reactive power present at the supply point.
If power consumption is measured on the high-voltage side, then it is sensible
to use individual compensation (no-load reactive power) of the feeder
transformer using a fixed stage.
If deviations from the applied setpoint occur, it switches capacitors on or off in
stages.
A base load can be programmed in automatic capacitor banks to compensate
series-connected transformers.
Benefits of central compensation:
Automatic adjustment of the compensation power to the
momentarily required active power.
Chapter 3
Exploitation of the simultaneity factor of the loads; thus less
capacitor power necessary.
Characteristics and functionality of compensation equipments
Important for planners and practitioners
More economic, since minimised use of separate components,
especially in the case of reactor protected systems.
Power capacitors
l
l
l
Adaptation to changing conditions by simple extensibility.
l
Light, simple, economic monitoring due to centralised layout.
l
Selection of the most beneficial compensation type
Economic and technical system aspects must be considered when deciding
whether the individual loads can be compensated most beneficially with fixed
capacitors or with central automatic capacitor banks. One can assume that
central compensation is 1.3 - 1.5 times more expensive than individual
compensation for the same power. If one however takes into account that in
most companies, it is rare that all loads are in operation simultaneously, lower
capacitor power is sufficient for central compensation. Both types of
compensation are already equally priced at a simultaneity factor of approx.
0.70.
Central compensation offers a further advantage particularly in the case of
reactor protected capacitors, since the high protection classes often needed
for fixed capacitors can be achieved only at extremely high extra cost in the
case of reactor protection.
A combination of different types of compensation can be worthwhile under
certain technical system conditions, e.g. to compensate large loads running for
long periods individually, in order to reduce the loading on transmission lines,
and the remaining reactive current of the other loads centrally at the main
distribution board. Several automatic capacitor banks, e.g. at main distribution
and sub-distribution boards or at controlled large consumers, are also
conceivable for similar reasons. One speaks of mixed compensation in such
cases.
Power capacitors should be employed wherever technically possible for
reactive power compensation and for the reduction of harmonics, due to a
higher price-performance ratio than achieved by other solutions.
They distinguish themselves by:
l
Simple, low-priced technology
l
Long service life and low maintenance cost
l
High operational security
l
Low losses
l
See Chapter 4 – “Capacitor technology” for more details.
Connections and protection equipment
Connection lines in and to compensation systems and their protection
equipment should be designed to continuously carry 1.5 times the capacitor
nominal current. This also applies to fuses, which should also have a time-lag
characteristic.
The ambient temperature and applicable regulations should also be observed.
Switching of capacitors
Connecting a single capacitor normally causes no problems, since the inrush
current is limited by the impedance of series-connected transformers and
cables (assuming satisfactory discharge before re-switching).
Switching a capacitor in parallel to capacitors that are already switched on is
significantly more critical. These supply a high inrush current, which is limited
only by the small impedance of the connection cables. Care must be taken
when switching capacitors in compensation systems to ensure that the
impedance between the capacitors is sufficiently high to reduce the inrush
current to reasonable values.
4
This is achieved in non-reactor protected systems by using special capacitor
switching contractors. These are equipped with precharging resistors as a
permanent component of the contractors. The capacitor is first connected to
the network via these resistors by means of early closing contacts, before the
main contacts close. The inrush current is then only approx. 5% of the nonattenuated value.
Capacitors with a series-connected reactor can be connected to the network
without this precaution, since sufficient attenuation is normally achieved by
the reactor inductance. The inrush current surge lies between 10 and 20% of
the non-attenuated value.
Power-up is performed in dynamic automatic capacitor banks with thyristor
switches with the aid of a special control system. Thyristor switches switch
almost without interaction when the voltage is equal between input and output.
See Chapter 9 – “Compensation in mains with quick load changes” for more
details.
Operational reliability of compensation systems is largely dependent upon the
quality and load reserves of the employed switchgear.
Discharge devices
Discharge devices are necessary to discharge capacitors to an uncritical
residual voltage after disconnection from the network voltage and before
reconnection.
They must be suitable to absorb and allow the energy stored in the capacitor
We = ½ CU² to decay within a period of seconds. One achieves this with
corresponding resistor combinations, special discharge modules or discharge
reactors. Discharge reactors allow rapid discharge with low losses.
To comply with IEC60831-1, every capacitor unit must be capable of reliable
discharge within three minutes to a residual voltage of <= 75 V.
As a basic rule:
The residual capacitor voltage when reconnected must be no higher than 10%
of the capacitor rated voltage. This must be taken into account especially for
switched capacitors for individual compensation and for automatic capacitor
banks. The controller reconnection time should be at least 10% longer than the
capacitor discharge time.
Connection to the supply network with phase opposition to the insufficiently
discharged capacitor must be avoided under all circumstances.
Achievable reconnection times using different techniques:
A reactive power controller detects the momentary reactive power
requirement by means of voltage and current measurements in the feeder to the
consumer's installation (distribution or sub-distribution). The capacitor power
is adjusted by connection or disconnection in such a manner that a target cos
programmed at the regulator is reached.
To avoid the reconnection of capacitors that have not yet been discharged,
certain switching or lock-out times are specified or preselectable in the
reactive power regulator, which are to be taken into account when choosing
discharge devices.
The distribution of the total power among the switched regulator control taps
(control outputs) is specified with the control series. It contains the number of
steps and their power ratings as a factor of the power of the first step, which is
defined as 1. The cross-sum of the control series gives the number of
combination steps of the compensation system.
For example:
Control series with four control steps of the same power: 1:1:1:1, the
compensation system is regulated in 1+1+1+1 = 4 combination steps.
Or:
All steps are provided with double the power of the previous step: 1:2:4:8, the
compensation system is regulated in 1+2+4+8 = 15 combination steps.
In this manner, it is possible to implement reactive power compensation
systems, with which the number of steps is significantly higher than the
number of control outputs at the reactive power controller.
Reactive power compensation systems are normally provided with 6 to 20
combination steps. The graduation is too coarse with smaller numbers of steps,
while more steps do not offer a significant improvement of cos .
In practice, one should choose control series up to 1:2:4:4:4 ... .
In this manner, a controller achieves with 4 control steps and a control series
1:2:4:4 = 11 combination steps and a controller achieves with eight control
steps and a control series 1:2:4:4:4:4:4:4 = 27 combination steps. Steeper
control series (e.g. 1:2:4:8:16...) should remain an exception.
Controllers with 8 control steps fulfil all practical requirements. If one wishes
to utilise the wear optimisation facility (loading back method) in modern
controllers, then each partial power should be provided at least twice in the
control series to allow a change between capacitor taps.
A relatively fine grading for 250 kVAR total power could, for example, be
arranged as follows:
1 : 1 : 2 : 2 : 2 : 4 : 4 : 4 = 20 combination steps
An increasing number of applications require significantly shorter
reconnection times.
See Chapter 9 – “Compensation in networks with quick load changes” for more
details.
Regulation, number of steps and control series
Most consumer installations are compensated using automatic capacitor
banks due to the recognised benefits. The compensation power is designed
with switched steps to match the employed capacitor power to the fluctuating
load situation in the consumer's system.
12.5 : 12.5 : 25 : 25 : 25 : 50 : 50 : 50 = 250 kVAR
or a little cheaper with one free tap but fewer change possibilities:
1 : 1 : 2 : 4 : 4 : 4 : 4 : 0 = 20 combination steps
12.5 : 12.5 : 25 : 50 : 50 : 50 : 50 : 0 = 250 kVAR
Systems with such graduated control series can be set up more economically
than systems with control series 1:1:1:1… and they function more effectively
with modern controllers.
6
Several capacitor contractors can, of course, be controlled from one control
output.
l
High quality controllers and automatic capacitor banks should:
l
Be connectable without attention to phase, phase relation and
current direction
l
Should calibrate themselves and become operational immediately
with minimal adjustment
l
Safely prevent that capacitors are reconnected before they have
been discharged to an uncritical value
l
Have a load dependent control characteristic and correct reactive
power load fluctuations as quickly as possible, while
protecting the installation and network, by direct switching
l
Use the installed capacitor power optimally by separately
adjustable target and alarm cos
Possess a facility (in the case of high voltage measurement) to
program a base load for the compensation of the seriesconnected power network transformer
l
Be simple to operate and possess clear display logic, which shows
all important network and installation data
l
Evaluate all relevant network and installation data and generate
an alarm if a danger of overload occurs and, if necessary,
disconnect endangered capacitor taps or the whole system
l
Store critical maximum values such as power network voltage and
harmonic loading (3rd, 5th, 7th, 11th harmonics)
l
There is no alternative: Quality pays for itself, not only for the controller,
but for all components in the automatic capacitor bank.
Reactor protection
Most automatic capacitor banks employed today are provided with reactor
protection as a result of the increasing harmonic loading of the consumer
installation and the power networks.
Every capacitor or capacitor tap is connected in series to an inductance
(reactor), in contrast to "normal" unprotected compensation.
If the resonant frequency of the series resonant circuit formed in this way
deviates by more than 10% from the frequency of the nearest harmonic, then
one speaks of a detuned resonator circuit. Reactor protected compensation
systems are designed as detuned resonator circuits and the series resonant
frequency f0 is normally chosen to be below the frequency of the 5th harmonic
(250 Hz). The capacitor and reactor system is therefore inductive for all
harmonic frequencies >= 250 Hz and dangerous resonance between the
capacitor and network inductance (e.g. transformer) is therefore avoided.
Consumer installations with high 3rd harmonic (150 Hz) components are an
exception but it can become necessary to set the series resonant frequency to
134 Hz in such cases.
If the series resonant frequency is less than 10% below or above a harmonic
frequency, then it is described as a tuned resonator circuit.
Tuned resonator circuits are normally employed as wave traps for the
deliberate reduction of individual harmonics.
Tariff switching
Some supply networks require that cos be limited to a certain maximum
Reactor protection-factor p
The reactor-protection factor p [%] specifies the ratio of the reactor reactance
to the capacitor reactance at network frequency.
value, e.g. 0.85 or 0.9 during low-load periods. A controller with tariff
switchover is necessary in such cases, which allows separate target cos to
The resonant frequency of the series resonant circuit can also be calculated
from p using the following equation:
be specified for high-load and low-load periods. This is activated by an external
signal.
1
fR =
f1 ×
p
For example: p=7 %, f1 = 50 Hz
Control quality
Increasing harmonic load, more sensitive manufacturing processes and
economic considerations in consumer systems designed only for the active
component pose ever increasing demands on the reliability and safety of
automatic capacitor banks. Besides this, their network interaction should be
as low as possible:
l
If the automatic capacitor bank fails, the main switch could trip
and production be interrupted. Claims for compensation due to
excessively high reactive power costs are also becoming more
frequent.
l
If the automatic capacitor bank does not switch back due to a
controller defect or incorrect wiring, a dangerous capacitive
overload can occur during low-load periods.
l
Serious damage can occur if the automatic capacitor bank is
overloaded by harmonics, for example, and it does not switch off.
The safety standards demanded today cannot be offered by a 100 € controller
and the 100 € saving is quickly swallowed by the first maintenance trip to the
customer - without mentioning the loss of confidence.
f
R
1
=
50 Hz ×
=
189 Hz
0.07
One of the often-tried standard values is normally used for the choice of a
suitable reactor-protection factor for the application:
The selection of the correct reactor protection prevents inadmissible
influences from the compensation system on any existing audio-frequency
ripple control system and reduces existing harmonics.
See Chapter 7 – “Compensation in mains with harmonics” and
Chapter 8 – “Compensation in mains with audio-frequency ripple control
systems” for more details.
7
Capacitor rated voltage with reactor protection
A voltage increase arises at the capacitor from the serial connection of the
reactor and capacitor. It can be calculated from the reactor-protection factor
p:
1
UC =
UNet ×
1p
The controller manufacturer's correction factors should be observed at other
voltages.
If a capacitor is requested for fixed compensation of the feeder transformer,
then it is to be installed in front of the current transformer.
The correct installation position for the current transformer
For example: p = 7%, UNet = 400 V
1
430 V
UC =
400 V ×
=
1 - 0.07
The capacitors employed for p = 7% must therefore be suitable for a continuous
rated voltage of at least 430 V.
Retrospective reactor protection
The voltage increase at the capacitor is a reason why old unprotected automatic
capacitor banks cannot be protected by reactors retrospectively. The higher
current associated with retrospective reactor protection would often also
exceed the switching capability of the contractors.
Unprotected capacitors that have been installed in networks for a long period
normally show considerable capacitance losses. The capacitance for reactor
protection should not deviate from the nominal capacitance by more than
± 5%.
Voltage and current measurement
The reactive power controller requires both voltage and current measurements
to determine the load conditions. An approximately symmetrical network is
normally assumed and a single-phase measurement system employed. This is
not normally a restriction, since the capacitor power is usually provided only
with three-phase switching.
The voltage can be measured at the compensation system power terminals. A
current transformer (secondary current preferably 5 A) must be inserted in the
feeder line of the distribution / consumer installation to measure the current.
Modern reactive power compensation systems permit the installation of the
current transformer in any phase, without taking the current direction into
consideration, with the help of an automatic calibration phase. Only the
installation position of the current transformer must be correct. It is installed
in such a manner that, as seen from the supply network, it is located in front of
all loads and also in front of the compensation system terminals.
The rating of the current transformer is chosen to suit the maximum current
in the feeder to the consumer installation to be compensated. Excessively rated
current transformers impair the control accuracy. If the phases are loaded
unequally, the current transformer should be installed in the phase with the
medium loading.
If other measuring instruments are to be connected to the same current
transformer, then the current paths of both devices are to be connected in
series (observe maximum loading VA).
The cross-section of the measuring leads should be 2.5 mm² for lengths up to
10 m, and 4 mm² for longer lengths.
The power ratio of the smallest capacitor step in kVAR, divided by the
CT transformation ratio should be >= 0.05 at 400 V.
controller
The most frequent errors can be quickly determined, since modern controllers
provide corresponding error signals.
If the current transformer is installed in the feeder to the loads, the reactive
power controller cannot measure the load current change and an error signal is
output.
The reactive power controller cannot detect a load current change and outputs a
signal when the current transformer is installed in the feeder to the
compensation system.
Summation current transformers
In the case of consumer installations with two or more feeders to one busbar
system or several busbar sections with continuously closed bus coupler
switches, the current in each feeder is to be measured using current
transformers. The secondary currents of the individual current transformers are
evaluated by means of a corresponding summation current transformer, which
can be installed in the compensation system, and passed on to the reactive
power controller. An unused primary circuit of a summation current transformer
must remain open.
8
One can normally do without the significantly more expensive high-voltage
measurement.
Compensation of the transformer using a fixed step or the base-load facility
integrated into the compensation system, in conjunction with optimal lowvoltage compensation, is sufficient to achieve a very good result in most cases.
If several busbar sections can be operated with both closed and also open bus
coupler switches and with one or several transformers, a compensation
system could be necessary for each busbar section. A differential
measurement is sensible in such cases, e.g. with three main current
transformers and two summation current transformers.
Chapter 4
Capacitor technology - a comparison
In the past, one could assume a service life of at least ten years for power
capacitors. Today, capacitor failures are often observed after significantly
shorter operation periods.
According to our experience, there are three causes for this:
Increased ambient temperatures due to increased packing density
in switchgear and equipment rooms.
l
Load increases due to harmonics, higher network voltages and
higher switching frequencies.
l
Use of low-priced capacitors, in which unacceptable
compromises with respect to materials, technology and field
strength are frequently accepted.
l
This circuit guarantees an optimal reactive power compensation for all circuit
or load configurations.
High voltage measurement
It is sometimes requested in consumer installations with high-voltage current
measurement that the measurement point for the reactive power
compensation also be located on the high-voltage side. In these cases, both
the voltage measurement and the current measurement must be made at the
high voltage current transformers.
The power ratio of the smallest capacitor step in kVAR, divided by the
CT transformation ratio and the transformer transformation ratio should be
>= 0.05 at a secondary voltage of 400 V.
The controller manufacturer's correction factors should also in this case be
observed at other voltages.
One of the most important factors affecting capacitor service life is the ambient
temperature. If it is increased by 7°C, for example, then the service life is
shortened to approximately half.
Self-healing
The vast majority of capacitors employed today for reactive power
compensation are self-healing, low-loss capacitors. The following comparison
is therefore deliberately restricted to this technology.
An evaporation layer of a special mixture of aluminum and zinc, or of pure
aluminum, is applied under high vacuum to a base material (Bi-axially oriented
polypropylene) for use in self-healing capacitors.
Self healing Power capacitors do not require series-connected fuses to protect
against short-circuits.
If breakdown occurs in the capacitor following voltage surges, the metallic
coating around the puncture point evaporates as a result of the temperature of
the arc that forms between the electrodes. The metal vapour is pressed away
from the centre of the puncture within several microseconds by the pressure
formed by the breakdown. A coating free zone is formed around the puncture
point, which is completely insulated.
The capacitor remains fully functional during the self - healing process. The
capacitance reduction is negligible.
9
Capacitor winding, enclosure, safety
The metal-coated base materials are then wound, around a tubular core, with or
without a further film (dielectric), depending on the technology employed, and
then contacted at the butt ends by metal spray techniques (Schoop process).
This is a highly complicated process done on state of the art machines in our
factory.
The completed windings are then wired up, dried in a complex process with
vacuum and heat and inserted into an aluminum, steel or plastic enclosure
under vacuum. This capacitor can is provided with a filler material in a further
manufacturing process to protect the contents against environmental
influences (oxygen and moisture). The enclosure is then hermetically sealed.
Long-tested safety system
Every capacitor eventually reaches the end of its useful life. Every capacitor can
be overloaded by unforeseen events or it can have defects in exceptional cases.
Only an effective pressure relief device can then help. The lid arches, interrupts
the power supply to the winding at the rupture joint. Only the combined function
of pressure release and disconnection from the power network guarantees the
necessary safety.
Safety system Rupture joint
Lid arch
Characteristics of a high-quality power capacitor:
A design that dissipates dangerous internal pressure and reliably
disconnects the capacitor winding from the power network after a
fault.
Rupture joint
A large number of the described self-healing events can occur during long
overloads or at the end of the capacitor's service life. Since self-healing
actions are always accompanied by gas formation and increases in internal
pressure, reliable safety precautions are necessary to avoid explosion of the
enclosure.
l
Gaseous or fluid, ecologically compatible, filler material with
long-term stability.
l
Three individual windings for three-phase capacitors to separate
the different potentials safely and to dissipate heat in an optimal
manner.
MKP capacitors
The winding consists of a low-loss bi-axially oriented polypropylene film,
coated on one side with an aluminium and zinc, which serves simultaneously as
the base material for the coating and also as dielectric.
l
Reliable sealing system with long-term stability to protect the
wrapping against oxygen and moisture.
Contacting
Large number of switching cycles by means of optimal butt-end
contacting and reliable internal connections.
l
Winding of an MKP capacitor
Metallized polypropylene film
Metallized polypropylene film
l
High capacitance constancy over the whole service life to avoid
dangerous drifting of the series resonant frequency in reactor
protected compensation systems.
l
Shock-hazard protected connection and discharge techniques with
high contact stability
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Design of an MKP capacitor
Advantages:
Small dimensions, high specific capacity
l
Lower price than Metallised Paper or MKV technology
l
High voltage load capability
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High impulse capability
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Low dissipation losses
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MKP capacitors are divided into three groups ac¬cording to the filler material:
Solid encapsulated MKP capacitors
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1. Case crimped seam
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2. Clamping
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3. Wire
4. Break-off release
(Rupture joint)
5. Wrapping
Liquid filled MKP capacitors
Gas filled MKP capacitors
Solid encapsulated MKP capacitors
The wrappings are normally inserted into plastic tanks after drying and
encapsulated with resin, which hardens completely.
Advantages:
Especially economic due to simple technology
l
Operation possible in any position
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No escape of liquid filler after a fault
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Relatively low disposal cost
l
10
Disadvantages
No effective pressure release possible, therefore additional
burst protection is necessary, e.g. by steel sheet enclosure.
l
Normally no effective long term protection of the wrapping
against oxygen or moisture since penetration possible
through connecting wires and hairline cracks in the resin.
Disadvantages
Very high demands on sealing and production quality
l
Greater care necessary during further installation work
l
l
The harmful influence of oxygen during the manufacturing
process not totally avoidable.
l
Liquid filled MKP capacitors
The wrappings are normally inserted in aluminium cans and vacuum dried by a
special process.
The cans are immediately filled with a non-hardening resin or a specially
treated vegetable oil and then sealed.
It is possible to fill the can with vegetable oil (flooding) directly in the vacuum
system, which offers the highest degree of protection.
Advantages
Reliable pressure release
l
Additional burst protection unnecessary
l
Very good permanent protection against oxygen and moisture
l
Very good heat dissipation
l
Influence by oxygen or moisture not possible with optimal
manufacturing process
l
Disadvantages
Operation recommended only in vertical position
l
Escape of liquid filler possible after a fault
l
Separation of solid and liquid components necessary for
disposal
l
Gas filled MKP capacitors
The wrappings are normally inserted into aluminium cans, vacuum dried by a
special process and flood¬ed with a protective gas, e.g. non-toxic and
physiologically safe nitrogen, while still in the drying container and then sealed
immediately.
This technology places very high demands on the quality of the employed
materials and the manufacturing process to achieve the necessary long term
stability.
Advantages
Reliable pressure release
l
Additional burst protection unnecessary
Summary
Disputes over the advantages and disadvantages of the different technologies
always arise in practice. We master all described technologies and can
therefore make recommendations free of constraints caused by material
interests.
Most importantly: All of our three-phase capacitors are equipped with three
separated wrappings and pressure release features.
We employ MKP capacitors with gas or vegetable oil filler, preferably for
unprotected and reactor protected compensation systems under standard
conditions.
We have come to the conclusion after careful consideration that the gas filled,
reinforced MKP capacitor, without mounting restrictions, with reliable pressure
release features and completely harmless filler material offers the best priceperformance ratio for applications ranging from standard to difficult
conditions.
We recommend high-quality 276 series or MKP UHD capacitors for reactor
protected compensation systems employed under difficult to extreme
operating conditions, such as high ambient temperature, very high switching
frequency and large voltage spikes, since they achieve long service lives under
such conditions.
Both technologies have advantages and disadvantages. Design and material
quality, monitoring and control of the whole manufacturing process, from
coating to final test, are crucial for long and reliable operation.
We adopt a conservative position on further size reduction and the associated
increase of field strengths - for your safety.
In spite of this, new coating techniques and structures, in association with
further process improvements will make even higher field strengths possible in
future.
One aspect always applies:
The user must pay attention to the technical parameters.
Capacitors for higher voltage levels
The design of capacitors for a higher voltage than the network voltage does not
protect against harmonics. Only reactor protection will help. High-quality
capacitors are designed generously with respect to voltage stability as a matter
of course.
Even the highest voltage stability cannot protect against resonance.
l
Good permanent protection against oxygen and moisture
l
Low weight
l
Operation possible in all mounting positions
l
Escape of liquid filling material impossible
l
Simple disposal
l
11
Chapter 5
Planning of compensation equipment
Important for planners and practitioners
Guidance values for the reactive power demand of motors and the
recommended capacitor power for individual compensation:
Individual compensation of transformers
Rated-
Only the no-load power should be compensated for feeder transformers. The
compensation power necessary for normal three-phase transformers is approx.
3 - 8% of the rated power. It should be compensated only when the power utility
requires it or in the case of consumption measurement on the high voltage side.
Guidance values for the reactive power demand of transformers and the
recommended capacitor power :
Individual compensation of welding equipment
Reactor protected fixed capacitors should, if possible, always be equipped with
a fuse and contactor to be able to use the reactor temperature sensors for
shut-down at overload.
Features of reactive power compensation for in-plant generation
equipment
Compensation of synchronous generators
The capacitor power is dimensioned for welding transformers at approx.
40 - 50% of the apparent transformer power.
Welding rectifiers require a capacitor power of approx. 10% of the apparent
device power.
The same conditions apply to welding converters as to motors.
Individual compensation of motors
The reactive power of the capacitor should not be greater than the no-load
reactive power of the motor, unless the capacitor is switched by its own
contactor (pay attention to discharge times). An excessive compensation power
can lead to impermissibly high voltages due to self-excitation during
deceleration. Fixed capacitors for motors with a star-delta switch and reverse
operation, with speed switchover, in lifts, hoists and brakes should always be
controlled by their own contactor to ensure clean isolation from the motor.
An automatically controlled system instead of a fixed capacitor can be
beneficial for large motors with considerable reactive load fluctuations.
The measures are restricted to the compensation of the consumer installation.
Precautions are necessary under certain circumstances to prevent voltage
increases due to capacitive operation (controller with high-speed
disconnection or voltage path via overvoltage relay).
Compensation of asynchronous generators
The capacitor power for fixed compensation should be approx. 35 - 50% of the
generator rated power.
Asynchronous generators may be operated in the network only in the unexcited
state.
The connect command for any fixed capacitors must therefore be given by an
auxiliary contact on the bus coupler switch.
Disconnection must be initiated simultaneously with the opening of the coupler
switch.
Capacitors with high voltage stability must always be used with contactor and
fuse. Automatic capacitor banks should be used with fluctuating power (water
or wind dependent). They should always be connected in front of the generator
switch (network side) in such a manner that the system can also be used to
compensate any loads during pure network operation. One can use 75 - 80% of
the generator power plus consumer component as a guidance value for the
capacitor power (measurement necessary). The current transformer is always
to be installed in the network feeder as already described. All loads, the
asynchronous generator and the compensation system must lie behind the
current transformer.
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In the case of changing energy directions - import or export - the target cos
must always be set to 1, to guarantee a symmetrical control characteristic in
both directions. A target cos setting of 0.9 inductive would become a target
Metal-halide lamps
cos of 0.9 capacitive after a reverse of the energy direction and thus cause a
dangerous over-compensation.
This precaution is not necessary for controllers that work on the four-quadrant
principle.
Compensation of power inverters, wind-power systems, photovoltaic
In-plant generation systems that are connected to the power network via power
inverters, e.g. photovoltaic systems and wind power systems, operate with a
cos between 0.5 and 0.7 (line-commutated inverters). They are normally
considerable sources of harmonics and are subject to large power fluctuations
due to clouds or wind variations. These special characteristics are to be
considered for the reactive power compensation; automatic control is
necessary. Capacitors and reactors are to be designed so that they can cope
with high voltage spikes and harmonic levels without capacitance losses over
the whole service life of this generating equipment. One should not accept
compromises with respect to voltage stability, especially for capacitors. Faults
in compensation systems for wind power plant can be very expensive.
We recommend series 275 or series 276 capacitors.
Sodium vapour lamps
Low-pressure sodium vapour lamps
Compensation is not necessary for self-commutated inverters, since their
cos is close to 1.0 .
Compensation of discharge lamps
Uncompensated discharge lamps with reactors operate with a cos of 0.3 - 0.6.
The following table gives guidance values for the capacitor power required for
discharge lamps with inductive ballast. The table restricts itself intentionally
to parallel compensation, since the series compensation loses significance
due to newer regulations.
A cos > 0.9 is achieved after compensation.
Discharge lamps
Warning:
Even lamp capacitors can be overloaded by harmonics, which can lead to
considerable consequential damage with unprotected capacitors. Only
protected capacitors should therefore be used in networks subjected to
harmonics.
Guidance values for average cos load-dependent
High pressure mercury vapour lamps
13
Guidance values for average cos -
the power utility and values of 33%, 50% or 62% (corresponding to cos
installation-dependent
0.95, 0.9 or 0.85) are usual.
One extracts the values for active and reactive work from the electricity bill for
the calculation of the necessary compensation power. Tan is calculated from
them.
Example:
Active work:
17,500 kWh
Reactive work:
21,000 kVARh
Target cos 1:
0.9
21,000 kVARh
=
1.2
tanj
1 =
17,500 kWh
Reduction of current and ohmic losses by installation of capacitors, offers
additional savings and safety
The corresponding cos = 0.64 is determined from the following table.
The row with the value 0.64 is selected from the cos 1 column. The column with
the target cos 2 = 0.9 is sought in this row and factor f = 0.72 is found at the
intersection.
The average active power at an assumed average monthly working period of
170 hours is:
P=
17,500 kWh
=
103 kW
170 h
The necessary capacitor power results from:
Q=
P×
f =
103 kW ×
0.72 =
74 kVAR
Guide values for average cos -
Including reasonable reserves, a compensation power of 100 kVAR should be
selected.
Load-dependent
Power tariff
The highest active power peak determined over, for example, 15 minutes is used
as the basis for the bill. An average cos is determined from the active and
Estimate of the compensation power using the transformer output
An approximate estimate of the necessary compensation power is possible on
the basis of the transformer output power at a new installation or if a
calculation basis is lacking.
Target -cos
reactive work measured over the month. An (assumed) apparent power peak is
calculated from these values, which is then billed. We recommend therefore
that a target cos = 1 is striven for.
The values for the peak active power and the average cos are taken from the
electricity bill.
Example:
Peak active power:
175 kW
Average cos :
0.7
A factor of f = 1.02 for the compensation of cos 1 = 0.7 to cos 2 = 1 is found in
the table Conversion factor f from cos 1 to cos 2 (Page 17).
Determination of the compensation power based on the electricity bill
The necessary compensation power is calculated as follows:
Determination of the compensation power with the help of the electricity bill is
the best solution, since the consumption situation over several months can be
taken into consideration. If extensions can be foreseen, these should also be
taken into account (guidance values can be found in the above tables).
Q=
P×
f =
175 kW ×
1.02 =
179 kVAR
With a reasonable reserve capacity, a compensation power of 200 kVAR is
recommended.
Kilowatt-hour tariff
The monthly active and reactive work is billed separately. For this, the
calculated reactive work is reduced by a free component of, for example, 50%
of the calculated active work. This free component is dependent upon
Determination of the compensation power by measurement
If an electricity bill is not available, a network ana¬lysis over a representative
month is recommended. It can also clarify any network problems.
We would gladly perform this work for you, please contact us.
14
Calculation table for the determination of the capacitor power
Conversion-factor f from cos1 to cos2
15
Fuses and incoming cables
Fuses and incoming cables for power capacitors at +30°C max.
Fuses and incoming cables for automatic capacitor banks at +30°C max.
Fuses and incoming cables for wave traps and wave trap equipment at +30°C max.
**
The currents quoted are the maximum permissible harmonic currents. The rating of fuses and incoming cables will include the 50 Hz fundamental current plus
the harmonic current. For intermediate sizes always use the next higher power step.
Disclaimer: The values mentioned above are for guidance only and non-binding. Regional regulations must be observed. Electronicon Kondensatoren GmbH is not
resposible for any error or ommission.
Selectivity – fuse
When the cos value is very poor, it can happen that the necessary fuse for the compensation equipment equals or exceeds the rating of the distribution or main fuse.
In order to avoid the selectivity problem in such cases, the compensation equipment can be spread over multiple cabinets with separate back-up fuses and feeders.
Another alternative would be to work with separate compensation units in the sub - or main distribution systems.
16