Presentation on theme: "QUALITY AND TECHNOLOGY quality of the. When used in electrical installations where harmonics are present, the application of a conventional capacitor."— Presentation transcript:
When used in electrical installations where harmonics are present, the application of a conventional capacitor bank is likely to suffer and cause several problems: 1) Failure and short lifetime of power capacitors 2) Elevated voltage harmonics 3) Danger of resonance between capacitor bank and main transformer HARMONIC PROBLEMS IN CAPACITOR BANKS Common problems: Capacitor banks provide power factor correction to correct low cos-phi. Nowadays more and more harmonic generating loads are used (frequency converters, UPS, soft starters, etc.) causing elevated levels of harmonics.
Problem 1: Failure of capacitors Impedance Z cap Frequency f fundamental 3rd harmonic5th harmonic Z cap = 1 /(2 f C) Current through the capacitor I cap = V / Z cap Z1Z1 The current through the capacitor depends on the impedance of the capacitor. The impedance of the capacitor is much lower for higher frequencies (harmonics). Therefore harmonics in the voltage cause elevated additional harmonic currents through the capacitor which overload and damage the capacitor Z3Z3 Z5Z5 Example: 3% of 5th harmonic in voltage causes 15% additional current through capacitor HARMONIC PROBLEMS IN CAPACITOR BANKS
DISTRIBUTION TRANSFORMER CAPACITOR BANK HARMONICS OF THE SAME FREQUENCY RESONANCE Problem 2: Elevated voltage harmonics L C The capacitance C of the capacitor bank and the inductance L of the main transformer and cables form an L-C circuit with certain resonance frequency. In case the resonance frequency is close to one of the present harmonics, this harmonic will be amplified, especially in the voltage. Harmonics cause overheating and malfunction of equipment. Example: 30% of harmonics in the current, will normally cause around 3% harmonics in the voltage. However a capacitor bank may increase the voltage harmonics to 8% or more. Tip: To know the effect of the capacitor bank on the harmonics, compare measurements of harmonics with and without connected capacitor bank.
Problem 3: Resonance DISTRIBUTION TRANSFORMER HARMONICS OF THE SAME FREQUENCY L C In case the resonance frequency of the LC circuit formed by the capacitor bank and the main transformer coincide with a present harmonic in the installation, a complete resonance can occur. The resonance may have destructive results for the capacitor bank, main transformer and main switchboard. DESTRUCTIVE RESONANCE Note: Resonance is a serious risk which shall not be under estimated, especially in installations with low impedance. HARMONIC PROBLEMS IN CAPACITOR BANKS CAPACITOR BANK
Frequency f fundamental3rd harmonic5th harmonic Impedance |Z| Resonance frequency Z3Z3 Z5Z5 Z = 2 f L + 1 /(2 f C) L C Benefits: -High impedance for harmonic frequencies: avoids high harmonic currents through the capacitor -Inductive impedance above resonance frequency: avoids amplification of harmonics and risk of resonance Z1Z1 HARMONIC PROBLEMS IN CAPACITOR BANKS Capacitive impedance Inductive impedance The filtering inductor provides a high impedance for high frequencies (harmonics). The total impedance of the capacitor + inductor step is inductive above the so-called resonance frequency Solution: Filtering inductors RTFX RTLX filtering inductor Total current I = V / Z total L + C
Total current I = V / Z total L + C Frequency f fundamental3rd harmonic5th harmonic Impedance |Z| Resonance frequency Z3Z3 Z5Z5 The resonance frequency is normally chosen at 189Hz (p=7%), however, in case significant 3 rd harmonics are present, it is recommended to choose 135Hz (p=14%) Z = 2 f L + 1 /(2 f C) L C The resonance frequency depend on the capacitor C and the inductor L, it is very important that both components are well tuned to each other. A wrong resonance frequency may result in harmonic absorption causing damage. Both capacitance and inductance values must therefore be correct and stable to guarantee correct tuning Z1Z1 HARMONIC PROBLEMS IN CAPACITOR BANKS Technical considerations 1 RTLX filtering inductor Capacitive impedance Inductive impedance
Filtering inductors cause a higher voltage on the capacitor terminals. The capacitors should therefore be suitable for this higher voltage. L C Note: Filtering inductors are recommended for electrical installations where the level of voltage harmonics THD%(V) is 2,5%..3% or more HARMONIC PROBLEMS IN CAPACITOR BANKS Technical considerations 2 Over voltage: 189Hz: 7,5% 135Hz: 14% Examples, for a 400V, 50Hz network: Inductors tuned to 189Hz (p=7%): Use capacitors for 440V, 460V, 480V, 500V or 525V Inductors tuned to 135Hz (p=14%): Use capacitors for 480V, 500V or 525V The effective kvar rating of the capacitor + inductor is calculated as follows: 189Hz: P_eff = (V_line/V_cap) 2 x 1,075 x P_cap 135Hz: P_eff = (V_line/V_cap) 2 x 1,14 x P_cap Example:line: 400V, 50Hz, capacitor: 440V, 50kvar inductor tuned to 189Hz: P_eff= (400/440) 2 x 1,075 x 50 = 44,4 kvar
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Line voltage400V, 50Hz Capacitor voltage440V, 50Hz Resonance frequency189Hz IsolationsClass F WindingsCopper, class HC-200ºC TerminalsCopper cable terminals Test voltage3 kV (1 min, 50 Hz) Harmonic currentsI3=8%, I5=31%, I7=13% Thermal overload factor5% Over current6% Tolerance3% Protection degreeIP-00 Ambient temperature40ºC Temperature rise<90ºC StandardsIEC/EN/UNE-EN 60289, CE IncludesBimetal temperature sensor, 120ºC, nc Standard characteristics Other characteristics on request (please indicate line voltage / frequency and capacitor rating / voltage / frequency) HARMONIC PROBLEMS IN CAPACITOR BANKS POLYLUX, S.L. C/ Boters 3b Parc Tecnològic del Vallès 08290, Cerdanyola del Vallès (Barcelona) Spain tel.: +34-936926565 fax: +34-935809603 firstname.lastname@example.org www.polylux.com
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