Why are Capacitors Connected in Series with Power Transmission Lines?

While capacitors are often associated with shunt applications for power factor correction, series capacitors are widely used in transmission lines to enhance voltage regulation, increase power transfer capacity, and improve overall system performance. In this article, we will explain why capacitors are connected in series in power transmission lines and what are the benefits and disadvantages.

A capacitor bank is connected in series with a power transmission line to compensate for the inductive reactance of the line. It effectively reduces voltage drops along the line, increases power transfer capability, and improves voltage regulation. Essentially, the capacitor’s reactive power counteracts the inductive reactance, allowing the system to transmit more power efficiently over long distances.

Inductive Reactance in Transmission Lines

High-voltage transmission lines inherently possess inductance due to their conductor diameter, length, and spacing. Excessive inductive reactance causes voltage drops along the line, limits current-carrying capacity, reduces power transfer capability, and degrades system stability.

This inductance creates inductive reactance (X_L = 2πfL), which opposes the flow of alternating current (AC). As the length of the transmission line increases, the inductive reactance grows, leading to:

• Increased voltage drops across the line.
• Reduced power transfer efficiency.
• Increased phase angle difference between sending and receiving end voltages.

To counteract these effects, capacitors are connected in series with the transmission line.

Related Post: Capacitor Bank in kVAR & µF Calculator for Power Factor Correction

Role of Series Capacitors in Power Transmission

Capacitors are connected in series in power transmission lines primarily for voltage compensation and power transfer enhancement. This practice, known as series compensation, helps improve the efficiency and stability of long-distance power transmission.

Series capacitors provide reactive power compensation by introducing capacitive reactance (X_C = 1 / (2πfC), which counteracts the inductive reactance of the line. This results in several benefits:

1) Compensation of Inductive Reactance

Since inductive reactance is positive and capacitive reactance is negative, they cancel each other out when capacitors are placed in series. This reduces the total effective reactance (X_eff) of the transmission line:

X_eff = X_L − X_C​

A lower effective reactance leads to reduced voltage drops and increased transmission efficiency.

Excessive inductive reactance causes voltage drops along the line, reduces power transfer capability, and degrades system stability. To counteract this effect, series capacitors are used to introduce capacitive reactance (X_C), given by:

XC = 1/ωC = 1/2πfc

Where “C” is the capacitance  and X_C in capacitive reactance provided by the capacitor.

2) Improvement in Power Transfer Capability

According to the power transfer equation:

Where,

• P = Power transferred
• V₁ = Sending voltage
• V₂ = Receiving voltage
• X_L = Inductive reactance of transmission line
• δ = Phase angle between V₁ and V₂

By reducing the excessive reactance X_L, series capacitors increase the power transfer capability of the same line. This way, it allows more power to be transmitted over longer distances.

With series compensation capacitor, the power transfer capacity becomes:

With series compensation capacitors, the power transfer capacity is enhanced as the capacitive reactance generated by series capacitors cancels out part of the inductive reactance of the transmission line. This effectively reduces the net impedance, enhancing power transfer efficiency. As a result, approximately 50% more power can be transferred when using series compensation devices.

3) Voltage Stability and Regulation

Transmission lines have inherent inductive reactance due to their length and the nature of conductors. While excessive inductive reactance causes voltage drops along the line, capacitors connected in series with transmission lines introduce capacitive reactance (-X_C), which cancels out part of the inductive reactance (+X_L). This reduces the overall impedance of the line and improves voltage levels at the receiving end.

This way, series capacitors help maintain voltage levels across long transmission distances, especially under high-load conditions. Without compensation, voltage drops due to line inductance may lead to instability and inefficient power delivery. By counteracting inductive voltage drops, series capacitors enhance voltage stability.

4) Improvement of Transient Stability

During disturbances (e.g., faults, load changes), transmission lines experience oscillations in power flow. Series capacitors enhance transient stability by increasing the synchronizing power coefficient, reducing the likelihood of system instability and blackouts.

Additionally, power oscillations can occur due to system disturbances. Series capacitors help stabilize these oscillations by altering power flow dynamics and improving transient stability.

5) Reduction in Ferranti Effect

In very long transmission lines, the receiving end voltage can become higher than the sending end voltage under light-load conditions due to excessive charging currents and line capacitance. This effect is known as “Ferranti effect“. Series capacitors offset this effect by balancing the reactive power in the system.

In case of Ferranti effect, appropriate reactors (inductors) and capacitators are added to the circuit to bring back the value of receiving end voltage to make it equal to the sending end voltage.

To counteract the Ferranti effect, appropriate reactors (inductors) and capacitors are added to the circuit to regulate the receiving-end voltage.

Conversely, when the receiving-end voltage is lower than the sending-end voltage, capacitors are used to boost the voltage. This process is known as static VAR compensation, where a fixed capacitor bank is connected in parallel with inductors to improve the power factor.

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3. Applications of Series Capacitors in Transmission Lines

Series capacitors are commonly used in:

Disadvantages of Series Capacitors

Despite their advantages, series capacitors have certain drawbacks:

• High Transient Voltages: Fault currents can cause high transient voltages across capacitors, requiring bypass circuits. The rating of series connected capacitor should be sufficient to continuously carry the full load current. This is because very high voltage are produced across the capacitor while clearing the fault current.
• Overvoltage Risks: Sudden changes in system conditions can cause overvoltage across capacitors, necessitating protective schemes such as Metal Oxide Varistors (MOVs). The rated capacitor must withstand short-circuit currents until protective devices operate properly to clear faults. High voltages across capacitors during fault clearing can cause capacitor failure.
• Limited use in DC Systems: Due to potential capacitor failures during short circuits, shunt capacitors are preferred over series capacitors. Series capacitors are rarely used in DC systems to mitigate inductance-related issues.
• Sub-Synchronous Resonance (SSR): Interaction between series capacitors and rotating machines (like turbines and generators) can cause Sub-Synchronous Resonance (SSR), a phenomenon where resonance below the system frequency (50/60 Hz) leads to mechanical damage, such as rotor shaft cracks and and unwanted resonance with harmonics effects.
• Maintenance Requirements: Series capacitor banks require regular monitoring and maintenance to prevent failures.

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