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Power capacitor types

Standard power capacitor units for power factor correction in medium-voltage networks are manufactured in single-phase and three-phase configurations, with standardized reactive power ratings ranging from 50 kVAR to 400 kVAR (in steps of 50, 100, 150, 200, 300 and 400 kVAR). These capacitors must withstand severe service conditions: continuous operation at 110% of the rated peak voltage and 120% of the rated RMS voltage, as well as overloads of 135% of the reactive power (kVAR) and 180% of the rated RMS current. The maximum allowable ambient temperature for continuous service is typically set at 55 °C / 131 °F (class D per IEC 60871), with a lower range of −40 °C / −40 °F in cold climates. Three main construction variants are distinguished: single-phase double-bushing unit (dead tank), single-phase single-bushing unit (enclosure as terminal), and three-phase three-bushing unit. All incorporate internal discharge devices that reduce the residual voltage to 50 V or less within a specified time.

Comparative Characteristics Table

Section titled “Comparative Characteristics Table”
Unit Type Bushing Configuration Typical Rated Voltage Range Typical Reactive Power per Unit Typical Basic Insulation Level (BIL) Grounding Cooling
Single-phase double bushing 2 insulated bushings 1 kV – 36 kV 50 kVAR – 400 kVAR 95 kV (for 15 kV) / 125 kV (for 25 kV) Dead tank (no ground connection) Natural (air)
Single-phase single bushing 1 bushing, metal enclosure as second terminal 1 kV – 25 kV 25 kVAR – 200 kVAR 75 kV – 125 kV One terminal grounded (live tank) Natural (air)
Three-phase three bushings 3 bushings (no neutral) 1 kV – 24 kV 100 kVAR – 600 kVAR (complete bank) 95 kV – 150 kV Isolated or system-grounded Natural (air) or forced

Typical Applications and Key Features

Section titled “Typical Applications and Key Features”

Single-phase double-bushing capacitors, designed to operate up to 110% of the peak voltage and 120% of the rated RMS voltage, are used in power factor correction banks where complete tank isolation is required (dead tank configuration). This variant is the most common in medium-voltage installations (e.g., 2.4 kV to 34.5 kV) for fixed and automatic compensation. Single-bushing capacitors reduce cost by using the enclosure as the ground terminal, ideal for systems with solidly grounded neutral and unit ratings up to 200 kVAR. Compact three-phase units, which can handle banks up to 600 kVAR, simplify mounting in industrial applications with limited space.

In all types, the internal discharge device ensures that the residual voltage drops below 50 V in less than 5 minutes (typically 3 minutes). The ability to withstand transient overcurrents is verified by short-circuit tests, with the requirement to withstand peaks of 100 times the rated current for 1 second. Insulation levels (BIL) range from 60 kV (for 5 kV systems) to 200 kV (for 36 kV systems); these values must be selected according to the rated voltage and the insulation coordination of the network.

Selection and Sizing Considerations

Section titled “Selection and Sizing Considerations”

Selecting a power capacitor requires verifying that the unit withstands at least 135% of the rated reactive power and 180% of the rated RMS current without premature degradation. The actual reactive power delivered to the system depends on the actual operating voltage according to the expression:

Qactual = (Voperating / Vrated)² × Qrated

Variable Description Unit
Qactual Reactive power delivered at operating voltage kVAR
Voperating Service RMS voltage kV
Vrated Rated voltage of capacitor kV
Qrated Rated reactive power of capacitor kVAR

Therefore, in networks with voltage fluctuations, it is recommended to specify the rated voltage of the capacitor 10% to 15% above the system nominal voltage, ensuring that the required reactive power remains within the tolerance margin. Additionally, ambient temperature directly affects service life: for every 10 °C / 18 °F increase above the maximum design temperature (e.g., 55 °C / 131 °F), the life expectancy is reduced by approximately half. At altitudes above 1000 m / 3281 ft, the heat dissipation capability decreases and a correction factor must be applied. Another critical parameter is the dielectric strength of the insulating material: modern capacitors use polypropylene dielectrics with a breakdown voltage on the order of 47 kV/mm / 1200 V/mil, allowing compact and reliable units even under temporary overvoltage conditions.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the maximum allowable overvoltage for a power capacitor?

Section titled “What is the maximum allowable overvoltage for a power capacitor?”

Power capacitors are designed for continuous operation at 110% of the rated peak voltage (1.1 × √2 × VRMS) and up to 120% of the rated RMS voltage, including harmonics but without exceeding the reactive power limit.

What standard kVAR values are used in the industry?

Section titled “What standard kVAR values are used in the industry?”

The standardized reactive power ratings per unit are 50, 100, 150, 200, 300 and 400 kVAR, although series/parallel combinations achieve banks of several MVAR.

Up to what temperature can a capacitor bank operate without loss of performance?

Section titled “Up to what temperature can a capacitor bank operate without loss of performance?”

The most common maximum design ambient temperature is 55 °C / 131 °F (class D), with a daily average temperature of 45 °C / 113 °F. The lower range can reach −40 °C / −40 °F in equipment prepared for cold outdoor environments.

How long does it take for a power capacitor to discharge to a safe level after disconnection?

Section titled “How long does it take for a power capacitor to discharge to a safe level after disconnection?”

Thanks to the internal discharge device, the residual voltage is reduced to 50 V or less within the time specified on the nameplate, typically 3 to 5 minutes.

What is the typical Basic Insulation Level (BIL) for a 15 kV system?

Section titled “What is the typical Basic Insulation Level (BIL) for a 15 kV system?”

For a 15 kV class system, the standard BIL is 95 kV (1.2/50 µs lightning impulse), which may be increased to 110 kV in heavily polluted environments or with special insulation coordination requirements.

What transient overcurrent capability must the units withstand?

Section titled “What transient overcurrent capability must the units withstand?”

The standard requires that capacitors withstand short-circuit peaks of up to 100 times their rated current for 1 second (or equivalent I²t values) without permanent damage.

References

Section titled “References”