Johnson & Phillips (part of MEM) medium and high voltage capacitors are electrically proven, single and three phase low loss units, manufactured in a wide range of ratings and enclosures suitable for indoor and outdoor installation.

All products comply with international standards and use environmentally friendly PCB-free impregnant.

Low voltage power capacitors utilise a "self-healing" dielectric consisting of a very thin plastic film onto which metallised electrodes are deposited by vaporisation. If a short circuit develops in the capacitor, the metal layer will vaporise in the area of the breakdown. This effectively cures the fault and the capacitor continues to function normally.

Conversely, high voltage capacitors employ a different technique wherein the dielectric comprises several layers of polypropylene film impregnated with an insulating fluid. Electrodes are made from thin aluminium foils. A high voltage capacitor is made up of a number of such elements connected in a series - parallel arrangement. The total number of elements is determined by the capacitors power rating and operating voltage. Each element is fuse protected.

After impregnation the completed assembly is hermetically sealed inside a steel container.

The Dielectric
The all-film dielectric used in high voltage capacitors is manufactured from the most advanced polypropylene film available which has the following benefits:

Reduced dielectric loss Decreased variation of capacitance with ambient temperature, to obtain a more constant output Significantly improved capacitor performance at low ambient temperatures

The Impregnant
The impregnation fluid, "Jarylec C101", is a chemical structure of benzyl toluene and dibenzyl toluene which has been specially formulated for power capacitors. It has been selected for both its physical and electrical properties and its high resistance to extremes of temperature.

The fluid is non chlorine biogradable and contains no PCBs.

Chemical Structure

Internal Element Fuses
Modern power capacitors are designed to withstand high stresses where the electrical gradient in the dielectric is typically in excess of 65 volts per micron. The capacitors in our range incorporate highly efficient back-up protection so that each individual element is safeguarded by a built-in, series connected fuse. In the event of a localised breakdown of the dielectric, only the faulty element is disconnected by the fuse and any subsequent reduction in capacitor output is minimised.

Capacitor units with only externally connected fuse protection have a distinct disadvantage. Faulty elements will continue to draw current which is marginly increased, whilst being of insfficient magnitude to rupture an external fuse until a succession of similar faults occurs. During this build-up time, the gases generated by arcing can cause sufficient pressure within the container to rupture the casing. With Johnson & Phillips capacitors, however, this risk is eliminated.

Discharge Resistors
These are installed within the capacitor assembly to reduce residual voltage from square root 2 x rated voltage to a value below 75V, within 10 minutes.

Capacitor Case
The stainless steel container is treated with a primer and finish coat of paint to suit the application.

Capacitor Bushing
Depending on requirements, one, two or three weld type porcelain bushings are fitted to the container. These have impulse withstand ratings of 75kV, 95kV or 150kV according to application.

Standards
IEC 871-1, IEC 871-2. Shunt capacitors for ac power systems having rated voltage above 1000V.

With the +15% capacitance tolerance the maximum possible current is 1.50 IN

Overvoltages having a higher value than 1.15 UN are not permitted more than 200 times in the life of the capacitor.

Capacitors must not be operated at both maximum voltage and current levels.

Temperature
Capacitors can be supplied for use in ambient temperatures ranging from
-50°C to +55°C.

The limits of ambient air temperatures are therefore:

Switching Voltages
A capacitor can only be energised when the residual voltage is below 10% of the rated voltage. The energisation of a capacitor bank by a restrike-free circuit breaker can cause a transient overvoltage, the first peak of which does not exceed 2 sq. rt. 2 times the applied voltage (rms value) for a maximum duration of ½ cycle.

It is assumed that the capacitor will not be switched more than 1000 times a year under these conditions.

Switching Currents
The peak value of the inrush currents must not exceed 100 times the rated current of the capacitor. In practice this value often needs to be limited to a lower figure dependent upon the switching device.

Overload Protection
This should be designed to protect the capacitor bank against overloads created by both overvoltage and overcurrent. With the modern low loss capacitor, the risk of thermal breakdown is minimal and any failure more usually results from dielectric breakdown. This occurs as a result of overvoltage which, in many cases, is caused by excessive harmonic current. Protection can be achieved by means of peak value voltage sensitive relays or current relays having characteristics designed to prevent overvoltage.

Undervoltage Protection
Where capacitors are connected to a network fitted with fast reclosing breakers, they must be fitted with undervoltage protection to ensure disconnection at voltage drop-out. They must not be reconnected until the residual voltage has fallen below 10% of the rated value.

External Fuse Protection
External fuses can be used to provide protection for individual capacitor units and the smaller capacitor bank. The selected fuse rating must take account of overvoltages, overcurrents and switching transients such that spurious operation is avoided.

Unbalance Protection
The purpose of unbalance protection is to disconnect a faulty bank before overvoltages greater than 10% appear on remaining healthy units. To achieve this, the capacitor units are arranged in two parallel star connections with the two star points being connected through a current transformer. Any fault occuring will result in current flow in the transformer which can be applied to operate a protective relay.

Damping Reactors
When switching a capacitor bank, high current transients appear. These can be particularly high if the bank is connected in parallel with another bank which is already energised. It may, therefore, be necessary to limit this inrush current by connecting a reactor in series with the capacitor bank. These transient current values can be calculated as follows:

Switching of a single capacitor bank

	IS = IN
where	IS = crest of inrush bank current, expressed in amperes
	IN = rated capacitor bank current (rms), expressed in amperes
	S = short circuit power (MVA) at the point where the capacitor is to be connected
	Q = the capacitor bank size expressed in Mvar
Switching of a capacitor bank in parallel with an energised bank(s)
	IS = Uwhere XC = U² . 10-6
where	IS = crest of inrush bank current, expressed in amperes
	U = phase to earth voltage, expressed in volts
	XC = series connected capacitive reactance per phase, expressed in ohms
	XL = inductive reactance per phase between the banks, expressed in ohms
	Q1 = outputs of the bank to be switched in
	O2 = the sum of the already energised bank(s), expressed in megavars

Enclosures
A complete range of enclosures is available to suit all the applications, selected from:

• Ingress Protection ratings from IP22 to IP55
• Indoor or outdoor use
• Cubicles, tanks or open racks
• Wall or floor mounting

Accessories
A complete range is available including the following:

• Controlling switchgear for manual or automatic control
• Voltage transformers
• Current transformers
• Current limiting reactors
• Detuning and filter reactors - iron and air-cored
• Out-of-balance protection systems

Lightning Arresters
In networks with switching or lightning overvoltages greater than 2.5 times the rated peak value, then lightning arresters (ZnO) are recommended in addition to overvoltage protection.

General
The products shown in this web site are typical examples only from the wide range available. Johnson & Phillips (Capacitors) can also build assemblies to match customers' precise requirements. Its expertise extends to the design and manufacture of capacitor banks used in extreme conditions such as those found in the frozen arctic of Northern Canada to the tropical climates of Africa and the Far East.

1) Customer connections

2) Termination chamber

3) HRC fuses

4) Undrilled gland plate

5) Earth stud

6) Capacitor units

7) Removeable panel