Basics of Power Factor Correction
Under normal operating conditions certain electrical loads (e.g. induction motors, welding equipment, arc furnaces and fluorescent lighting) draw not only active power from the supply, but also inductive reactive power (kvar). This reactive power is necessary for the equipment to operate correctly but could be interpreted as an undesirable burden on the supply. The power factor of a load is defined as the ratio of active power to apparent power, i.e. kW, kVA and is referred to as cosφ. The closer cosφ is to unity, the less reactive power is drawn from the supply:
If cosφ = 1 the transmission of 500kW ina 400V three phase mains requires a current of 722 A. The transmission of the same effective power at a cosφ= 0.6 would require a far higher current, namely 1203 A. Accordingly, distribution and transmission equipment as well as feeding transformers have to be derated for this higher load. Futhermore their useful life may also be decreased.
For systems with a low power factor the transmission of electric power in accordance with existing standards results in higher expenses both for the supply distribution companies and the consumer.
Another reason for higher costs are the losses incurred via heat dissipation in the cables caused by the overall current of the system as well as via the windings of both transformers and generators. If we assume for our above example that with cos φ= 1 the power dissipated would amount to about 10kW, then a power factor of 0.6 would result in a 180% increase in the overall dissipation i.e. 28kW.
In general terms, as the power factor of a three phase system decreases, the current rises. The heat dissipation in the system rises proportionately by a factor equivalent to the square of the current rise.
This is the main reason why Electricity Supply Companies in modern economies demand reduction of the reactive load in their networks via improvement of the power factor. In most cases, special reactive current tariffs penalize consumers for poor power factor.
- A reduction in the overall cost of electricity can be achieved by improving the power factor to a more economic level.
- The supply will be able to support additional load which may be of benefit for an expanding company.
- Reducing the load on distribution network components by power factor improvement will result in an extension of their useful life.
Types of Power Factor Correction
A capacitive reactive power resulting from the connection of a correctly sized capacitor can compensate for the inductive reactive power required by the electrical load. This ensures a reduction in the reactive power drawn from the supply and is called Power Factor Correction.
Most common methods of power factor correction are:
SINGLE OR FIXED PFC, compensating for the reactive power of individual inductive loads at the point of connection so reducing the load in the connecting cables (typical for single, permanently operated loads with a constant power).
GROUP PFC - connecting one fixed capacitor to a group of simultaneously operated inductive loads (e.g. group of motors, discharge lamps)
BULK PFC, typical for large electrical systems with fluctuating load where it is common to connect a number of capacitors to a main power distribution station or substation. The capacitors are controlled by a microprocessor based relay which continuously monitors the reactive power demand on the supply. The relay connects or disconnects the capacitors to compensate for the actual reactive power of the total load and to reduce the overall demand on the supply.
A typical power factor correction system would incorporate a number of capacitor sections determined by the characteristics and the reactive power requirements of the installation under consideration. Sections of 12.5 kvar, 25 kvar, and 50kvar are usually employed. Larger stages (e.g.100kvar and above) are achieved by cascading a number of smaller sections. This has the beneficial effect of reducing fluctuations in the mains caused by the inrush currents to the capacitors and minimizes supply disturbances. Where harmonic distortion is of concern, appropriate systems are supplied incorporating detuning reactors.
Calculation of Required Capacitor Power
The reactive power which is necessary to achieve a desired power factor is calculated by the following formula:
QC = P • F
QC - reactive power of the required correcting capacitor
P - active power of the load to be corrected
F - conversion factor acc. to chart 1
|Conversion Factor for a Target Power Factor|
a.Consumption of active energy: ………EW = 300000kWh
b.Consumption of reactive energy: ………EB = 400000 kvarh
c.Number of working hours: …………t = 600 h
|Resulting average active power:||P =||= 500kW|
Calculation of the original power factor cosφ1:
For the improvement of the power factor from 0.6 to 0.9 we read the factor 0.849 from the chart above.
The required capacitor power is: QC = 500kW•0.849 = 425 kvar
Influence of Harmonics, Harmonic Filtering
Developments in modern semiconductor technology have led to a significant increase in the number of thyristor - and inverter-fed loads.
Unfortunately these non-linear loads have undesirable effects on the incoming AC supply, drawing appreciable inductive reactive power and non-sine-wave current. The supply system needs to be kept free of this harmonic distortion to prevent equipment malfunction.
A typical inverter current is composed of a mixture of sinewave currents; a fundamental component at the supply frequency and a number of harmonics whose frequencies are integer multiples of the line frequency (in three phase mains most of all the 5th, 7th, and 11th harmonic). The harmonics lead to a higher capacitor current, because the reactive resistance of a capacitor reduces with rising frequency.
The rising capacitor current can be accommodated by constructional improvements in the manufacture of the capacitor. However a resonating circuit between the power factor correction capacitors, the inductance of the feeding transformer and the mains may occur. IF the frequency of such a resonating circuit is close enough to a harmonic frequency, the resulting circuit amplifies the oscillation and leads to immense over-currents and over-voltages.
Harmonic distortion of an AC supply can result in any or all of the following:
- Premature failure of capacitors.
- Nuisance tripping of circuit breakers and other protective devises.
- Failure or maloperation of computers, motor drives, lighting circuits and other sensitive loads.
The installation of detuned (reactor-connected) capacitors - see pic. 1 - is designed to force the resonant frequency of the network below the frequency of the lowest harmonic present, thereby ensuring no resonant circuit and, by implication, no amplification of harmonic currents. Such an installation also has a partial filtering effect, reducing the level of voltage distortion on the supply, and is recommended for all cases where the share of harmonic-generating loads is more than 20% of the overall load to be compensated. The resonance frequency of a detuned capacitor is always below the frequency of the fifth harmonic.
A close-tuned filter circuit however is tuned to a certain harmonic frequency and presents avery low impedance to the individual harmonic current, diverting the majority of the current into the filter bank rather than the supply.
DEFINITIONS AND SELECTION CRITERIA
Rated Voltage UN
Root mean square of the max. permissible value of sinusoidal AC voltage in continuous operation.
The rated voltage of the capacitors indicated in the data charts must not be exceeded even in cases of malfunction. Bear in mind that capacitors in detuned equipment are exposed to a higher voltage than that of the rated mains voltage; this is caused by the connection of detuning reactor and capacitor in series. Consequently, capacitors used with reactors must have a voltage rating higher than that of the regular mains voltage.
The voltage at a detuned capacitor's terminals can be calculated as follows:
UN = Rated mains voltage
UC = Capacitor voltage
p = Detuning factor
Test Voltage Between Terminals UBB
Routine test of all capacitors conducted at room temperature, prior to delivery. A further test with 80% of the test voltage stated in the data sheet may be carried out once at the user's location.
Voltage test between terminals and case UBG
Routine test of all capacitors between short-circuited terminals and case, conducted at room temperature. May be repeated at the user's location.
Rated Power QC
Reactive power resulting from the ratings of capacitance, frequency, and voltage.
Current Rating lN
RMS value of the current at rated voltage and frequency, excluding harmonic distortion, switching transients, and capacitance tolerance.
Maximum RMS Current Rating lMAX
Maximum rms value of permissible current in continuous operation. The maximum permitted rms current for each particular capacitor is specified in the data charts and is related to either construction features or the current limits of the terminals. In accordance with IEC 831 all ELECTRONICON capacitors are rated at least 1.3 x lN, allowing for the current rise from permissible voltage and capacitance tolerances as well as harmonic distortion. The exact value for each capacitor can be found in the data charts. Higher rms values than stated in the data charts require adjustments in construction and are available on request.
Continuous currents that exceed these values will lead to a build-up of heat in the capacitor and - as a result - reduced lifetime or premature failure. Permanent excess current may even result in failure of capacitor's safety mechanisms, i.e. bursting or fire (see pg. 21).
Care must be taken not to exceed the maximum voltage and current ratings when installing capacitors in close-tuned or detuned equipment (see data sheets for maximum ratings). The thermal monitoring of reactors, or the use of overcurrent protection relays in the capacitor circuit is recommended to protect against overloads.
Pulse Current Strength lS
Depending on construction and voltage rating, the design of our capacitors permits short term inrush currents of 100…400 x lN and - in accordance with IEC 831- up to 5000 switching operations per annum as standard. However, when switching capacitors in automatic capacitor banks without detuning reactors, higher loads are very often the case. This may have a negative effect on the operational life, especially of capacitors which are frequently connected and disconnected (e.g. primary stages).
We therefore strongly recommend the use of special contactors with inrush limiting resistors, or other adequate devices for limitation of the peak inrush currents.
The average useful life of a capacitor depends very much on the ambient temperatures it is operated at. The permissible operating temperatures are defined by the temperature class stated on label which contains the lower limit temperature (-40°C for all ELECTRONICON power capacitors) and a letter, which describes the values of the upper limit temperatures. The following chart details the maximum permitted ambient temperatures for capacitors for each temperature category acc. to IEC831-1.
|Temperature Category||Ambient Temperature Limits Maximum||Max. Average Over 24 Hours||Max. Average Over 365 Days|
MOUNTING AND OPERATING INSTRUCTIONS
Safe operation of the capacitors can be expected only if all electrical and thermal specifications as stated on the label, in the data sheets or catalogues and the following instructions are strictly observed.
ELECTRONICON does not accept responsibility for whatever damage may arise out of a non-observance.
Resin-filled MKP-276-capacitors shall be installed upright with terminals facing upwards. Gas-filled MKPg-275-capacitors can be mounted in any position without restrictions, however, a position with terminals pointing downwards shall be avoided!
The useful life of a capacitor may be reduced dramatically if exposed to excessive heat. Typically an increase in the ambient temperature of 7°C will halve the expected life of the capacitor.
The permitted temperature category of the capacitor (B, C or D) is stated on the label. If extenuating circumstances give cause for doubt, special tests should be conducted to ensure that the permitted maximum ambient temperature of the capacitor is not exceeded. It should be noted that the internal heat balance of large capacitors is only reached after a couple of hours.
To avoid overheating the capacitors must be allowed to cool unhindered and should be shielded from external heat sources. We recommend forced ventilation for all applications with detuning reactors. Give at least 20mm clearance between the capacitors for natural or forced ventilation.
Do not place the capacitors directly above or next to heat sources such as detuning or tuning reactors, bus bars, etc.
Vibration Stress According to DIN IEC 68-2-6
Please consult us for details of permitted vibration stress in your application. Note that capacitors fitted with the EL-Dr discharge reactor must not be exposed to any vibration stress at all.
All cylindrical capacitors can be fixed sufficiently using the mounting stud at the base of the can. It is recommended to insert the washer which is delivered together with the mounting nut before fixing the nut.
Fuses and cross section of the leads shall be sized for at least 1.5 times of the rated capacitor current (iN). Please note that the permitted max. current acc. to data (lMAX) must not be exceeded. Do not exceed the permitted max. current values per contact as specified in the chart below even when coupling capacitors in parallel.
Do not exceed the permitted torque of the terminal screws (design K, L,M) and the mounting studs. The test values specified by IEC must be guaranteed as a minimum value.
All cylindrical capacitors are fitted with a "break action" safety mechanism which may cause the case to expand, especially at the fold and at the lid.
- The capacitors shall only be connected with flexible or elastic copper bands.
- The folded crimps must not be held by retaining clamps.
- A clearance of at least 35mm above the terminations shall be accommodated.
Required clearances according to applicable voltage category must be maintained even after a prolongation of the can.
The hermetic sealing of the capacitors is extremely important for a long operating life and for the correct functioning of the beak action mechanism. Please pay special attention not to damage the following critical sealing points:
- the bordering of the lid
- the connection between screw terminal and lid (design K, L, M)
- the rubber seal at the base of the tab connectors (design A)
- the soldering at the base of the tab connectors (design A)
|A||6||16 each plug|
|10**||30||1.2 … 2.0|
|L||25*||43||2.5 … 3.0|
|50**||80||3.2 … 3.7|
|*(mit Ader-Endulse_with ferrule) **(ohne Ader-Endulse_without ferrule)|
Capacitors should be discharged to <10% of the rated voltage prior to being re-energized. For this purpose, special discharge modules are offered which can be selected in accordance with the applied operating voltage and the desired discharge period. Standard IEC 831 requires a discharge to 75V or less within 3 minutes. Note that in automatic capacitor banks shorter discharge cycles may be required.
Use rapid discharge reactors or switchable discharge resistors for very short discharge cycles. Capacitors must be discharged and short-circuited before working on the terminals.
For capacitors in design L/M, six separate discharge modules (3 x 68kΩ, 82kΩ, 100kΩ, 120kΩ, 180kΩ, 300k Ω)) are available for the discharge of single capacitors or groups of several connected capacitors. The resistors are allocated in a finger-proof housing (IP20).
The correct size of the module to be applied can be taken from the recommendations given in the capacitors data charts. The values recommended there have been designed for a discharge below 50V within no more than 60 seconds.
For design A capacitors, similar discharge sets are available (IP00). The correct size of the module to be applied can be taken from the recommendations given in the capacitor data charts. The values recommended there have been designed for a discharge below 50V within no more than 70 seconds.
Capacitors in design K are provided with internal discharge resistors for a discharge below 50V within no more than 60 seconds as standard.
Alternatively, the resistors to be used can be calculated with the following formula:
|t……Discharge period in (s)||UB……Operating voltage|
|CT……Partial capacitance of one phase||UE……Maximum permissible voltage after period t|
|CTOTAL……Total capacitance||R……Module resistance value|
|Three-phase capacitors||Single-phase capacitors|
|R =||R =|
The discharge resistors may become very hot (up to 200°C) during continuous operation!
For design L/M only: Remove the lid of the discharge module if applying protective caps to the capacitors!
Capacitors with a metal case must be earthed at the mounting stud or by means of a separate metal strap or clamp.
Our capacitors do not contain PCB, solvents, or any other toxic or banned materials. They do not contain hazardous substances acc. to <<Chemische Verbotsverordnung>> (based on European guidelines 2003/53/EG and 76/769/EWG), <<Gefahrstoffverordnung>>(GefStoffV) and <<Bedarfsgegenstaendeverordnung (BedGgstV)>>.
Not classified as <<dangerous goods>> acc. to transit rules. The capacitors do not have to be marked under the Regulations for Hazardous Goods. They are rated WGK 0 (water risk category 0 <<no general threat to water>>).
No danger for health if applied properly. In case of skin contact with filling liquids, clean with water and soap.
The impregnants and filling materials contain vegetable oil or polyurethane mixtures. A data sheet about the impregnant utilized can be provided by the manufacturer on request.
We recommend disposing of the capacitors through professional recycling centers for electric/electronic waste.
The capacitors can be disposed of as follows:
- Disposal acc to European Waste Catalogue 160205 (capacitors filled with plant oil/resin).
- Hardened filling materials: acc. To EWC 080404 (<<solidified adhesives and sealants>>).
- Liquid filling materials which may have emerged from the capacitor shall be absorbed by proper granules and disposed of in accordance with European Waste Catalogue 080410 (PUR resin residues, not solidified).
Caution: When touching or wasting capacitors with activated break-action mechanism, please consider that even after days and weeks these capacitors may still be charged with high voltages!
Consult your national rules and restrictions for waste and disposal.
Protection Against Overvoltages and Short Circuits: Self-Healing Dielectric
All dielectric structures used in our 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 relatively resistive and voltage proof for all operating requirements of the capacitor. The capacitor remains fully functional during and after the breakdown.
For voltages within the permitted testing and operating limits the capacitors are short-circuit-and overvoltage-proof.
They are also proof against external short circuits as far as the resulting surge discharges do not exceed the specified surge current limits.
Protection Against Accidental Contact
All capacitors are checked by routine test: voltage test between shorted terminations and caseUBG ≥2 x UN + 2000V (at least 3000V) in accordance with VDE 0560. Accessible capacitors must be earthed at the bottom stuff or with an additional earthling clamp.
The terminal block of designs K, L and M is rated IP20, i.e. it is protected against accidental finger contact with live parts. The discharge modules are designed in the same way. Unused contact cages of design M terminal blocks must be covered by a proper blank (available as standard accessory).
Capacitors in design A are not provided with protection against accidental contact as standard. They are available with protective caps on request.
Protection Against Overload and Failure at the End of Useful Service Life
In the event of overvoltage or thermal overload or ageing at the end of the capacitor's useful service life, an increasing number of self-healing breakdowns may cause rising pressure inside the capacitor. To prevent it from bursting, the capacitor is fitted with an obligatory <<break action mechanism>> (BAM).This safety mechanism is based on an attenuated spot at one of the connecting wires inside the capacitor. With rising pressure the case begins to expand, mainly by opening the folded crimp and pushing the lid upwards. As a result, the prepared connecting wire is separated at the attenuated spot, and the current path is interrupted irreversibly. It has to be noted that this safety system can act properly only within the permitted limits of loads and overloads.
MIND HAZARDS OF EXPLOSION AND FIRE
Capacitors consist manly of polypropylene (up to 90%), i.e. their energy content is relatively high. They may rupture and ignite as a result of internal faults or external overload (e.g. temperature, overvoltage, harmonic distortion). It must therefore be ensured, by appropriate measures, that they do not form any hazard to their environment in the event of failure or malfunction of the safety mechanism.
FIRE LOAD: appox. 40MJ/kg
EXTINGUISH WITH: dry extinguisher CO2, foam
MKP-MKPg-type capacitors are based on a low-loss dielectric formed by pure polypropylene film. A thin self-healing mixture of zinc and aluminum is metallized directly on one side of the PP-film under vacuum. Our long-term experience as well as on-going research and improvements in this technology ensure the excellent self-healing characteristics of the dielectric and a long operating life of our capacitors.
The plastic film is wound into stable cylindrical windings on the most modern automated equipment. The ends of the capacitor windings are contacted by spraying with a metal contact layer, facilitating a high current load and ensuring a low-inductance connection between the terminals and windings.
The link between PP-film and zinc contact layer is highly stressed during high surge or rms current and therefore considered very critical for operating life and reliability of the capacitor. By cutting the film for selected types in a wavelike manner, we increase the contact surface between film and zinc layer which substantially reduces this strain.
The use of filling materials in capacitors is necessary in order to insulate the capacitor electrodes from oxygen, humidity, and other environmental interference. Without such insulation, the metal coating would corrode, an increasing number of partial discharges would occur, the capacitor would lose more and more of its capacitance, and suffer increased dielectric losses and a reduced operating life. Therefore, an elaborate vacuum-drying procedure is initiated immediately after insertion of the capacitor elements into the aluminum case and dried insulation gas (MKPg 275), or biologically degradable plant oil (MKP 276), is introduced. Both protect the winding from environmental influence and provide an extended life-expectancy and stable capacitance.
MKPg 275 - Leakage Proof and Environment Friendly
The gas in our MKPg capacitors is inert and entirely harmless to environment. When disposing of the capacitors, no liquids or toxic gasses need to be considered.
A leakage of gas is extremely unlikely if the capacitors are handled and operated properly. It is possible to mount these capacitors in any desired position. However, should leakage occur, the leaking gas would escape into the atmosphere causing no undesirable effects to the adjacent equipment, e.g. damage, pollution, or staining. In the long run, such as unlikely event would result in a degradation of the capacitance; however, this process would take many months, during which the capacitor remains functional.
By using gas, we are reducing the weight of a capacitor on average by 15…20% compared with resin or oil filled capacitors. This makes transportation and handling of the units easier. It also supports the new concept of mounting the capacitors in almost any position.