Thermal Design of Automatic Power Factor

Correction Panels

Mudit Bhatia

Importance

• Temperature – A deciding factor of life, especially for capacitors in APFC panels

• For every 10° C increase in temperature, life of the capacitor (any dielectric) reduces to half

• Parameters affecting the internal temperature of the panel :

Ti = Ta + PL.ZTHwhere, Ti : Internal temperature of the panel

Ta : Ambient temperature (around panel)PL : Power loss of components inside the panelZTH : Thermal impedance of the panel (°C / W)

ZTH

PL

Ta

Ti

What are the factors affecting thermal impedance?

No sufficient space provided

around the reactor

1. Panel design related – Poor thermal design

Blocking/ Not providing suitable path for heat flow

Reactor kept near capacitor in

completely enclosed compartment

Insufficient sizing of fans (lower heat flow rate) and/or louvers

Exhaust fans with less CFM

1. Panel design related – Poor thermal design

Louvers

Cooling Fans

PLINTH

Inappropriate location of panel breathers/ fans

Louversimmediately under

exhaust fan

Inappropriate fan mounting

Reduced rate of air

flow, hence,

overheating

1. Panel design related – Poor thermal design

Capacitors kept above

reactors

Reactors kept one above another

Improper positioning of major heat sources inside the panel

Capacitors kept close to

reactors

1. Panel design related – Poor thermal design

2. Site Conditions Related

Dust Choking the breathers

Air velocity measurement through louvers

Choked with dust – no air flow

Air flow after cleaning

• Cooling Fan Failures • Modifications made by the end user, inside the panel after commissioning

Thermistor not connected

2. Site Conditions Related

Leakage through gasket due to aging

Louvers

Cooling Fans

PLINTH

Insufficient space around panels

• Panels kept too close to the walls

• Some objects blocking the bottom / inlet louvers

• Panel room ventilation

2. Site Conditions Related

Recommendations

Reduced rate of air flow,

hence, overheating

Louvers

Cooling Fans

PLINTH

Louvers

Cooling Fans

PLINTH

Use of rails or

perforated plates for mounting

• Decide the cooling method based on the allowed temperature rise

• Proper selection of fan size and number of such fans by doing necessary calculations

• Isolating the heat source with the temperature sensitive components

• The above can be enforced through new standard, which shall be derived through IEC 61439 / IEC 61921

Recommendations for better thermal design

Thank youתודה

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