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Terminology

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Rated Capacitance (C)

Designed capacitance of the capacitor at 20oC / 50 Hz to 120Hz

 

Capacitance Tolerance 

Admitted capacitance deviation from the rated capacitance.

 

Rated Voltage Un

The maximum direct voltage or the maximum r.m.s. alternating voltage (50 Hz) or the peak value of a pulse voltage which may be continuously applied to a capacitor at any temperature between the lower category temperature and the rated temperature.

 

Rms Voltage (Urms)

Root mean square of the maximum permissible value of the sinusoidal AC voltage in continuous operation.

 

Ripple Voltage (Ur)

Peak to peak alternating component of the unidirectional voltage

 

Non-recurrent Surge Voltage (Us)

Surge voltage induced by a switching or any other disturbance of system which is allowed for a limited number of times and for durations shorter than the basic period.  

 

Maximum Current (Imax)

Maximum rms current for continuous operation.

 

Maximum Peak Current (Ipeak)

Maximum permitted repetitive peak current that can occur during continuous operation. The value is following:

Ipeak = C * dv/dt.

C = Rated Capacitance

dv/dt = The rate of voltage rise, which means maximum permitted repetitive rate of voltage rise of operational voltage.

 

Maximum Surge Current (Is)

Peak non-repetitive current  induced by a switching or any other disturbance of system which is allowed for a limited number of times and for durations shorter than the basic period.  

 

Temperature Derated Voltage

The maximum voltage that may be continuously applied to a capacitor for any temperature between the rated temperature and the upper category temperature.

 

Operating Temperature Range

The operating temperature of the capacitor is defined as the ambient temperature + self-temperature raise + temperature rise due to thermal radiation from other heat sources.

 

Climatic Category

The climatic category which the capacitor belongs to is expressed in numbers (standard IEC 60068-1: For example 40/85/56).

40 = Lower Category Temperature -40 °C

85 = Upper category Temperature +85 °C

56 = the days relevant to the damp heat test 56days

 

Temperature Coefficient of Capacitance (α)

The change rate of capacitance with temperature measured over a specified range of temperature. It is normally expressed in parts per million per Celsius 

degree (10-6/°C) and referred to 20°C

α=(Ci−C0)/(C0(Ti−T0))

Ci = Capacitance at the temperature Ti

C0 = Capacitance at the temperature T0 (20±2) °C

 

Series Resistance (Rs)

Effective ohmic resistance of the conductors of a capacitor under specified operating conditions. It depends on temperature and the approximate TCR is 0.004/°C.

Rs(T2) = [1+0.004 * (T2 – T1)] * Rs(T1)

 

Equivalent Series Resistance (ESR)

ESR is the ohmic part of an equivalent series circuit. Its value assumes all losses to be represented by a single resistance in series with the idealized capacitor.

The ESR comprises the polarization losses of the dielectric material (Rpol), the losses caused by the resistance of the leads, termination and electrodes (Rs) and the insulation resistance (Ris)

ESR=tanδ/(ω*C)

 

Dielectric Dissipation Factor (tan δd)

Constant dissipation factor of the dielectric material for all capacitors at their rated frequency. The typical loss factor of polypropylene film is 2*10 -4.

 

Loss Factor of The Capacitor (tan δ)

The dissipation factor is ratio between reactive power of the impedance of the capacitor and effective power when capacitor is submitted to a sinusoidal voltage of specified frequency, it is  that ratio between the equivalent series resistance and the capacitive reactance of a capacior.

 

Impedance (Z)

The impedance Z is the magnitude of the vectorial sum of ESR and the capacitive reactance XC in an equivalent series circuit under consideration of the series inductance L.

Z= √(ESR^2+ (ωL+ I/ωC)^2 )

The impedance is typically measured on capacitors (radial types) having 2 mm long leads.

 

Insulation Resistance (Ris) and Time Constant (τ)

The Ris is the ratio of an applied DC voltage to the resulting leakage current (flowing through the dielectric and over its body surface) after the initial charging current has ceased. The Ris is typically measured after one minute ± 5 s at 20 °C and a relative humidity of 50 % ± 2 %.

 

 

The insulation resistance is determined by the property and the quality of the dielectric material and the capacitor's construction. The Ris decreases with increasing temperature. A high relative humidity may decrease the insulation resistance. Ris changes due to moisture are reversible. The Ris is shown as time constant (τ). It is the product of insulation resistance and capacitance and is expressed in seconds

τ = Ris * C

 

Inductance (L)

The inductance of a capacitor depends upon the geometric design of the capacitor element and the length and the thickness of the contacting terminals. All  film capacitors have an extended metalized film or foil construction and exhibit thus a very low inductance. The inductance of radial leaded capacitor types are typically measured with 2 mm long lead wires. Typical values are less than 1.0 nH per mm of lead length.

 

Dielectric Power Loss (Pd)

Loss power induced by dielectric polarization or dielectric conductance. The value is following:    

Pd = U² x π x f0  x C x tan δd

for DC capacitor:  U = Ur/2                                                     for AC capacitor:  U =√2 urms

for GTO snubber capacitors: U = Undc/ 2

f0 : fundamental frequency

C: capacitance

 

Joule Power Loss (Pj)

Loss power induced by series resistance of the capacitor under  rms current, the value is follwing:

Pj = I2rms x Rs

 

Capacitor Loss  (Pt)

Active power dissipated in the capacitor, consist of dielectric loss and joule loss. 

Pt =  Pd + Pj

 

Resonance Frequency (fr)

Lowest  frequency at which the impedance of the capacitor becomes minimum. The value is following:

fr = 1/(2π x√Ls x Cn)

 

Maximum Operating Temperature  (θmax)

The highest temperature of the case at which the capacitor may be operated. 

 

Minimum Operating Temperature  (θmin)

The lowest temperature of the case at which the capacitor may be energized. 

 

Cooling-air Temperature  (θamb)

Temperature of the air measured at the hottest position of the capacitor, under steady-state conditions, midway between two units. If only one unit is involved, it is the temperature of surrounding air, measured 10 cm away and at 2/3 of the case height of the capacitor under steady- state conditions.

 

Contained Temeperature Rise  (Δθcase)

Difference between the temeprature  of the  hottest point of the container and the temperature of the cooling air.

 

Thermal Resistance (Rth)

The thermal resistance indicates by how many degrees the capacitor temperature at the hotspot Rises above the ambient temperature per watt of the heat dissipation losses.

 

Hotspot Temperature  (θhs)

Temperature at the hottest spot inside the capacitor. the value is following:

θhs=θamb + Pt x Rth

 

Failure Rate ( λ )

It indicates the failure probability of components in unit time and the value is the number of failure components in unit time compared to total number of components, the unit of λ is FIT (also expressed as Fit or fit) and 1 Fit = 1/(109hrs)

For example 10 000 pcs of components work at given condition for 10000 hrs and 10 pcs of components failed, so λ=10/(10 000 x)=100 Fit

 

Self-Healing

Self-healing, also known as clearing is the removal of a defect caused by pinholes, film flaws or external voltage transients. The heat generated by the arcing during a breakdown, evaporates the extremely thin metallization of the film around the point of failure, thereby removing and isolating the short circuit conditions. On Segmented Film Technology Capacitors, the self-healing effect is more controlled. The film metallization is made by forming a pattern of segments, which are connected to each other by micro fuses. This limits the healing current and limits the self-healing effect to a well-defined section of the film.

 

Expected Lifetime of Capacitor

The expected life time of capacitor depend on the applied voltage and the hot spot temperature during the operation. For capacitors applied in different situation, the designed average service life is different. 

In the capacitor industry, capacitors used in DC-Link circuits will have an expected lifetime of probable 100,000 Hrs at rated voltage and 70 °C hot spot temperature.

Expected lifetime is a statistical value calculated on the basis of experience and on theoretical evaluations. The following diagrams show the correlation between expected life, operating voltage and hot spot temperature. The diagram should be considered only as a reference. Please contact our technical department if you have any further question.