This description focuses on single capacitors and reactive power compensation
systems in the power range from 50 to approx. 6,000 kvar, because the greatest
deficit in risk-awareness is present here. In practice, these PFC systems are
often just «parked» somewhere since the dimensions and prices of such equipment
are relatively small and they should be close to the load. Protection devices
are then often neglected in comparison to larger systems, because the costs
appear to be unreasonably high.
It is a fact that
the protection systems necessary for the reliable operation of a small reactive
power compensation system, e.g. 300 kvar, require the same protection relay and
a similarly expensive current transformer as a 10 MVAR system. If the total
power is divided into 3 or 6 units, even higher costs are incurred. Disparities
of up to 80% for the protection component then occur, compared to 5% for larger
systems.
Serious mistakes are often made especially
in the reactive power compensation of small installations, which lead to
equipment damage and environmental incompatibility. This unsatisfactory
situation was the starting point for the development of a new concept that
offers an acceptable relationship between protection costs and total costs,
while reducing the ecological risk.
There is a newer technology of metallised polypropylene film(MKP) for medium voltage which is becoming very popular and is much suited for such application compared to All film technology. Before we discuss the limitations of all film technology a some advantages of the MKP technology just to keep you interested.
Operational
stress of Allfilm medium voltage capacitors
Capacitors operate
at full power immediately after every switching. No-load or low-load periods
do not exist. The design is made under economical constraints with high
electrical field strengths up to 75 kV/mm and a finite service life, which can
be dependent on many influencing factors, and is estimated from statistical
data. There are many effects that cannot be detected during durability tests.
Summarizing, it can
be said that modern power capacitors are very reliable and failure rates
greater than 0.2% per year are very rare. However, it must be considered that
much higher failure rates occur from early failures, manufacturing faults,
dimensioning errors, incorrect application or overload.
The effects of such failures must be
assessed carefully by means of a risk analysis that also includes the
ecological risk, due to the high short-circuit power present in medium voltage
installations.
Objective of
capacitor protection techniques
The reduction of damage to the environment
is the most important criterion for capacitors, while preventive protection
against permanent damage is the prime concern with motors, transformers,
inverters, lines, cables and similar components.
Breakdown Behavior of Allfilm medium voltage capacitors
Allfilm capacitors are
constructed with layers of aluminum foil and polypropylene film. These foils
are first winded together and then flattened to make packets. These packets
are connected in series and parallel
connections inside a steel enclosure to make a capacitor. Finally, this whole
assembly is immersed in synthetic mineral oil (e.g. Jarylec) to complete the
construction. In some cases each packet has its own fuse which is meant to disconnect
the packet in case of failure.
Breakdown of the dielectric is the prime cause
of failure. Only this breakdown and the resulting consequential effects are
considered here.
Every
breakdown of a single winding element in a capacitor with several internal
serial winding circuits leads to a change of the internal voltage distribution,
irrespective of whether this winding is fused individually or whether it is a
fuseless design. This leads inevitably to higher voltage stress in the remaining
winding elements. Accelerated ageing accompanies the increased voltage stress,
which results in further winding
breakdowns.
Considerable damage to the environment of the
capacitor must be expected if the breakdown process is uncontrolled, i.e. if operation
is continued until an over-current, earth fault or short-circuit trip responds.
This means that, when the maximum permitted energy input into the
capacitor casing is exceeded (violation of the typical current-time
destruction limit), it can, in the worst case, tear open, and the contents of the casing can be ejected. A considerable
shock wave, with ignition of the oil spray and the solid flammable content, are
conceivable further consequences.
3-phase
medium voltage capacitors
These capacitors are frequently
built without internal fuses, due to their – usually - low power and
three-phase design. After a winding breakdown, such a capacitor develops a
group short-circuit.
Depending on the number n of
internal serial circuits, the capacitance and voltage stress in the affected
branch increases by the factor n/n-1.
The current increases correspondingly. Thermally, this
is barely noticeable due to the low capacitor losses. Further short-circuits
can occur, especially if the fault remains unnoticed for a long period.
When we have a failure
situation, the short-circuited capacitor element and all paralleled elements are
bridged out of operation. As a result, the effective kvar output of the capacitor
increases.
This is now 181kvar.
When an external star circuit is functioning with 3x 150kvar and one of the
capacitors suddenly becomes 181kvar, this causes shifting of the star point and
stresses in the circuit.
Consider an example with
fuse-protected
capacitors.
In this case, the effective kvar output comes down, but the voltage stress
on the functioning packets, in this case, increases by more than 19%.
This increased stress is disastrous for the
capacitor.
Internal
fuse Protected Capacitor
Internal series
connections depend on the voltage on the elements and the capacitors, because a
failure of one element causes overvoltage on the other healthy units. The
failure of one element may result in voltage rise between 0.5 to 2.5% depending
on the internal construction. For smaller voltages this may not be such a
problem but for higher voltages eg 7.2kV these fuses are under additional
stress and prone to rapid failure. Another problem is to have a fuse with the right
breaking capacity at the high voltage and to have sufficient open distance once
the fuse has operated.
When the element
fails the fuse may result in arcs and formation of Gas in the enclosure which
leads to contamination of the oil, however much of the manufacturers may claim.
The fusing temperature is often between 700~1000°C, which is well above the
flashpoint of the insulating oils (e.g. Jarylec has a flash point of 144°C).
The important point here is that there are numerous such fuses inside the
capacitor and therefore the chances of catastrophic failures are quite high.
Another important
aspect is that a failure of one element may cause a loss of capacitance from 2
to 5% depending on the internal construction in a capacitor. This is one of the
biggest advantages with MSD as we will see later in this note. MSD being based
on self-healing technology has no loss of capacitance even after a short fault
is cleared.
When there is short
circuit between the terminals the discharge current of the capacitor is very
high. Normally the manufacturers claim 200~300 times the rated current, but in
certain case may go up to even 1000 times the rated current. The internal fuses
are highly susceptible to failure during these times. When there is an internal
fault (element breakdown) , due to the lower circuit impedance inside the
capacitor, the discharge current from the elements connected directly in
parallel to the damaged element, is even higher than a direct external
discharge.
External
Fuse Protected Capacitor
Internally protected
capacitors can never be protected by an external fuse. Therefore, when one
element fails, not only the energy of the parallel elements discharges through
the fault but also of the other capacitor units in parallel. If the fuse does
not break quickly, usually an explosion or severe mechanical damage will occur.
Because a fault in a
group will only marginally increase the unit current, it is not easy to have a
rapid operation of the external fuse at power frequency. For example, with one element
being faulty may result in the current increase of only 30%. Therefore, some
groups may see permanent overvoltage of 30% over some months, and eventually
fail. Even during failure their currents are not high enough for a rapid
breakdown. At this time there might be arcing in the capacitor and a strong
possibility of rupturing or other sever mechanical damage and fire.
Dielectric structures used in our MSD power capacitors
are “self-healing”: In the event of a voltage breakdown the metal layers around
the breakdown channel are evaporated by the temperature of the electric arc
that forms between the electrodes. They are removed within a few microseconds
and pushed apart by the pressure generated in the centre of the breakdown
spot. An insulation area is formed which is reliably
resistive and voltage proof for all operating requirements of the capacitor.
The capacitor remains fully functional during and
after the breakdown. In the graphic above the black layers are the metal layers
(shown exaggeratedly which is normally in 10s of Angstroms) and the blue is the
Polypropylene between 4~8um. In the last sequence, it is shown how at the end
of the self-healing process the insulation area is formed. Normally this
insulated area is <2mm therefore there is negligible loss of capacitance.
Capacitance remains constant throughout
the life of the capacitor.
For voltages within the permitted testing and
operating limits the capacitors are overvoltage and short-circuit-proof.
They are also proof against external short circuits as
far as the resulting surge discharges do not exceed the specified surge current
limits.
Overpressure
Protection
Another distinct
advantage of self-healing capacitors is the implementation of Over Pressure
Protection mechanism. The self-healing process generates gases inside the
capacitor. In normal course because of the dry construction these gases stay
embedded in the resin. However when the capacitor nears its end of life failure
the number of self-healing breakdowns increases which causes the pressure to
rise inside the capacitor. MSD capacitors are equipped with an over pressure
switch which disrupts the control voltage of the contactor when there is a
pressure rise and thereby safely disconnecting the capacitor. It is impossible
to have such a safe system in Allfilm capacitors, which are prone to frequent
ruptures and explosions.
Below is a comparison of the Electronicon MSD(TM) capacitors versus the ALL film capacitors.
For more information please visit Electronicon website for more information.
