Common Myths Regarding Power Quality and Compensation-2
We continue with second part of our article series regarding power quality and compensation.
It is safer to select capacitor with 525V label value in a facility with 400V busbar voltage.
False. Busbar voltage and application type are considered when selecting the capacitor voltage value. However, selecting a capacitor with 525V nominal voltage for 400V busbar voltage means unnecessary huge voltage-power sizing and similar to selecting 4000A thermal magnetic instead of 1600 A in a facility having 1300 A maximum load current.
IEC 60831-1 Table-3 – Allowed Maximum Voltages
In filtered compensation applications, one further voltage reduction, namely voltage increase is realized due to a filter serial connected to the capacitor. This increase depends on impedance of the serial connected reactor and corresponds to approximately 7% rate (%p=%7) in a commonly used accord frequency. That brings the voltage on the capacitor in a busbar with 400V nominal voltage to 400 V x (1 + 0,07) = 428 V level. This increase shall be changed depending on change in %p value, namely the resonance frequency. Therefore, capacitor nominal voltage should be selected as 430 V or higher instead of 400 shall be enough in filtered compensation applications. Capacitors are manufactured as endurable against 10% (8s/24s) surplus of nominal voltages according to IEC 60831 standard. This endurance level increases up to 30% in short term voltage increases (1 minute). The table related with voltage endurance in IEC 60831-1 standard is as given right. That indicates the capacitor shall be able to work in any voltage increase.
Another important point regarding power quality and compensation is sizing with excessive power. Output power changes depending on the voltage implemented to capacitor. Let’s select a capacitor with 525 V nominal voltage in order to work in 400 V busbar and equip capacitors in this system with a 7% reactor. In this case, the power required to be used in 525V for acquiring 25 kVAr effective power in 400 V should be 40 kVAr. That indicates we can effectively use 62,5% of design power and 37,5% of power cannot used due to excessive sizing. A compensation system with 1000 kVAr 525 V 50 Hz power shall be able to give 625 kVAr power to connected 400 V 50 Hz grid. If 430 V capacitor was selected in this application instead of 525 V capacitor, 675 kVAr 430V 50 Hz compensation power shall be enough for acquiring 625 kVAr 400 V power. That means, design power is converted to output power at a high rate as 92,5%.
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