How System Voltage, Frequency, and Reactance Ratio Affect Capacitor Output Capacity

In Power Quality management, capacitors are widely used for reactive power compensation. However, the actual reactive power output (in kvar) of acapacitor often differs from its rated value. This blog explains how several key parameters—system voltage, frequency, and reactance ratio—influence capacitor performance.

1. Rated Voltage vs. System Voltage

Capacitor output is proportional to the square of the voltage:

Q = 2πf C V²

• For three-phase capacitors, the rated capacity usually refers to the total output of all Three Phases.
• In line-to-line connection (common compensation), the rated voltage is calculated using line voltage.
• In phase-to-neutral connection (split compensation), it's based on phase voltage.

Important Note: Capacitors must withstand at least 1.1 times the system voltage. For instance, in a 400V system:

400V × 1.1 = 440V

So, common capacitor rated voltages are 440V or 450V for three-phase units, and 250V for split-phase units.

2. Frequency Effect on Output Capacity

Capacitive reactance (Xc) is inversely proportional to frequency:

Xc = 1 / (2πfC)

As frequency increases, reactance decreases, and the capacitor’s reactive power output increases. For example, a capacitor rated at 10kvar @ 50Hz will output 12kvar @ 60Hz.

3. Reactance Ratio of Series Reactor

To suppress harmonics, capacitors are often paired with series reactors. The reactance ratio (e.g., 7%, 14%) directly affects:

1. Capacitor Working Voltage

Reactors increase the operating voltage on the capacitor. The actual required capacitor voltage becomes:

V_capacitor = V_system × 1.1 / (1 - Reactance Ratio)

Example: For a 7% reactor and 400V system:

V = 400V × 1.1 / (1 - 0.07) ≈ 473V → Choose a 480V-rated capacitor

1. Output Capacity Reduction

Reactors reduce output capacity slightly:

Q_reactor = Q_capacitor × Reactance Ratio

A 50kvar capacitor with 7% reactor → reactor capacity = 3.5kvar

4. Resonance and Harmonic Suppression

The tuning frequency of a reactor-capacitor system determines harmonic suppression ability:

• 7% Reactance → approx. 189Hz → suppresses 5th harmonic (250Hz) and above
• 14% Reactance → approx. 134Hz → suppresses 3rd harmonic (150Hz) and above

5. Intelligent Capacitor Units

Modern solutions like intelligent capacitors combine the capacitor, reactor, and switching module into a single unit. Key types:

• Standard Intelligent Capacitor: No built-in reactor
• Harmonic-Resistant Intelligent Capacitor: Includes a series reactor

They are classified by:

• Rated voltage (e.g., 450V, 250V)
• Rated capacity (e.g., 30kvar)
• Compensation type (Common or Split)
• Reactance ratio (e.g., 7%)

Summary Table

Need help choosing the right capacitor or reactor for your system?

Contact Intone-power, and let us help you improve your power quality with confidence.