Self Healing MV Capacitors Vs. All film Capacitors

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.