Cooling

During initial operation, it is strongly recommended to inspect your system using an infrared (IR) camera. This allows you to identify any hot spots, which may indicate issues such as inadequate cooling or localized heating caused by stray magnetic fields. Early detection of these thermal anomalies helps ensure safe, efficient operation and prevents long-term damage to components.

Cooling Requirements for Polypropylene Capacitors

Polypropylene capacitors can safely operate at internal temperatures up to 90°C. However, under full power, there is typically a temperature gradient of 45°C between the core of the polypropylene element and the copper surface of Celem capacitors. To prevent overheating, the external surface temperature must not exceed 40–45°C.

To maintain this temperature limit, the cooling water temperature at the outlet of the capacitor circuit should also not exceed 40–45°C.

Improved cooling performance can be achieved by either lowering the inlet water temperature or increasing the water flow rate.

For reference, a water flow of 1 litre per minute dissipating 1 kW of power will result in a temperature rise of 14°C, based on the specific heat capacity of water. This ratio can be used to calculate the required water flow, given the power loss and inlet water temperature.

Maximum inlet water pressure for Celem water-cooled capacitors is 5 bar.

When brazing a water tube to the busbar, it is recommended to position the tube as close to the capacitor as possible to maximize thermal efficiency.

When the capacitor bank and the work coil are cooled in series, it’s important to note that the work coil typically dissipates at least ten times more power than the capacitor bank. Therefore, to ensure efficient cooling and prevent overheating of the capacitors, the cooling circuit should be arranged so that the capacitor bank is cooled first, followed by the work coil.

Cooling at Derated Power Levels

This graph shows the relationship between electrode temperature and the percentage of nominal power applied to the capacitor.

As power increases, the internal hotspot temperature rises, requiring a lower external electrode temperature to maintain proper heat dissipation. At 100% nominal power, the maximum allowed electrode temperature is 45°C to ensure effective thermal management.

At reduced power levels, internal heating is significantly lower, which allows for a higher external electrode temperature—up to 85°C at 0% power—without compromising capacitor integrity.

This graph should be used as a general guideline rather than a strict rule. Keep in mind that for the same power level, higher current and lower voltage conditions tend to generate more heating than scenarios with lower current and higher voltage.

Use this chart with caution, as breakdown voltage decreases with temperature, and excessive local heating can compromise capacitor performance and reliability.

Capacitor Bank Losses

A capacitor bank consists of the capacitors themselves and the connecting elements between the capacitors and the output terminals. Power losses can originate from either component.

• Capacitor Losses: These are typically very low—approximately 5 × 10⁻⁴ of the reactive power and is a sum of dielectric and ohmic losses of the capacitors.
• Connection Losses: In a well-designed capacitor bank, connection losses are comparable to capacitor losses.

The total power loss of a capacitor bank is the sum of the capacitor and connection losses, and is generally around 10⁻³ of the reactive power.
We recommend estimating total losses in the range of 1/1000 to 1/700 of the reactive power for practical design and thermal considerations.

Copper Connections and Thermal Conductivity
All copper-to-copper connections must be made on flat surfaces to ensure optimal electrical and thermal conductivity. To improve surface contact, it is recommended to apply thermal paste or thick silicone oil between the contact points. When properly tightened, the paste or oil will fill only microscopic gaps and be displaced from the actual contact areas.

Both sides of a Conduction Cooled capacitor must be connected to water cooled bus bar.

The entire contact surface of the capacitor must be in full contact with the heat sink using thermally conductive paste to achieve efficient heat dissipation. This is especially critical when the capacitor operates near its maximum rated limits.

Mounting a CSM capacitor on the top of another CSM capacitor is not recommended.  It causes an overheating of the upper capacitors.

Conduction Losses of the busbar
Power capacitors can deliver hundreds of amperes. When multiple capacitors are connected to a common busbar, insufficient surface area—especially under high-frequency conditions—can lead to extreme heating of the busbar due to the skin effect. This may result in overheating of the capacitor despite adequate cooling. Proper sizing and layout of the collector surface are essential to minimize conduction losses.

Induction Heating in Capacitor Assemblies
When several conduction-cooled capacitors are mounted in parallel between two busbars, those positioned closest to the output terminals may experience additional induction heating. This is due to the concentration of current and magnetic field at the collection point, which affects the last capacitors in the path. To prevent this, careful busbar design is required (see illustrations below).

Celem’s coaxial capacitor technologies—such as C-CAP, Q-CAP, and V-CAP—eliminate this issue by enabling fully compensated busbar configurations, effectively isolating the capacitors from magnetic fields and associated induction heating.