What is a Series Reactor ?
A series reactor, also known as a line reactor, is an inductive component placed in series with a circuit to limit the current, control harmonics, and improve power quality. Its primary function is to oppose changes in current, providing inductive reactance that helps protect electrical equipment and stabilize the system.
Key Functions:
- Current Limiting: It restricts short-circuit current and protects equipment like transformers, motors, and switches.
- Harmonic Filtering: Series reactors are used to mitigate harmonic currents caused by nonlinear loads, improving power quality.
- Voltage Regulation: They help regulate voltage by limiting the inrush current during switching operations.
Table of Contents
Tuned Series Reactor:
A tuned reactor is used in harmonic filtering applications, where it is combined with capacitors to form a filter that targets and neutralizes specific harmonic frequencies.
Working Principle:
- Tuned reactors are connected in series with capacitorsto form a resonant circuit at a specific frequency, usually one of the harmonic frequencies.
- The combination of inductance (from the reactor) and capacitance creates a resonant circuittuned to a specific harmonic (e.g., 5th or 7th harmonic).
- At the tuned frequency, the reactor and capacitor trapor absorb the harmonic currents, preventing them from flowing into the power system.
- This helps in improving power qualityby reducing harmonic distortion, which can damage sensitive equipment and increase losses.
Applications:
- Tuned reactors are widely used in harmonic filter banksand power factor correction systems, especially in industrial facilities with significant nonlinear loads (e.g., variable frequency drives, large motors).
Detuned Series Reactor:
A detuned reactor is also used with capacitors in power factor correction systems, but instead of targeting a specific harmonic, it is designed to avoid resonance with the most common harmonics in the system.
Working Principle:
- Detuned reactors are used to prevent the resonance between the capacitor bank and the grid at harmonic frequencies.
- By adding a reactor in series with capacitors, the resonant frequency of the circuit is shifted below the harmonic range(typically below the 5th harmonic).
- This prevents the amplification of harmonic currents that could otherwise occur due to resonance between the network and the capacitors.
- The detuned circuit protects the capacitorsfrom being overloaded by harmonics and helps improve the overall power factor of the system without causing harmonic issues.
Applications:
- Detuned reactors are commonly used in power factor correction systemsto prevent harmonic amplification while improving power factor in industrial or commercial installations.
How Tuned and Detuned Reactors Works :
- Tuned Reactors are designed to filter out specific harmonics by forming a resonance at the harmonic frequency, allowing them to trap and neutralize harmonic distortion.
- Detuned Reactors are designed to prevent resonance and protect power factor correction capacitors from the damaging effects of harmonics. They shift the resonance point away from common harmonic frequencies, ensuring that the capacitors only correct power factor and do not contribute to harmonic amplification.
Both types of reactors play an essential role in maintaining the health of the electrical system by reducing harmonics, limiting inrush currents, and protecting equipment.
Why Series Reactors are Used with Capacitor Banks
Series reactors are commonly used with capacitor banks for several reasons, mainly to manage harmonics, control inrush current, and protect the capacitor bank itself:
- Harmonic Mitigation:
Capacitor banks are highly sensitive to harmonic frequencies generated by nonlinear loads like variable frequency drives, UPS systems, or industrial machines. Without series reactors, capacitor banks can resonate with the power grid’s harmonics, amplifying harmonic distortion in the system.- Series reactors (either tuned or detuned)are connected in series with the capacitors to filter out or avoid resonating with harmonic currents, thereby improving the overall power quality.
- Inrush Current Limitation:
When a capacitor bank is switched on, it can draw a large inrush current due to the difference in voltage between the capacitor and the system. Series reactors limit this inrush current, protecting the capacitors and other electrical equipment from damage. - Voltage Regulation and Protection:
Reactors reduce the risk of over-voltage conditions that may occur when capacitors are added to improve power factor. They also prevent capacitors from being overloaded by harmonic currents, thus extending the life of the capacitor bank.
Can Series Reactors Be Used Without Capacitor Banks?
Series reactors can be used without capacitor banks, but their application changes:
- Without Capacitor Banks:
- Current Limiting: In some cases, series reactors are used alone to limit short-circuit or fault currents in high-power systems, such as transformers or motors.
- Voltage Regulation: Series reactors can also stabilize voltage fluctuations in electrical grids or industrial systems.
- Motor Protection: In motor circuits, series reactors are used to limit starting currents and protect against sudden spikes.
However, when not paired with capacitors, the primary function shifts more towards current limiting, voltage regulation, or harmonic reduction in other scenarios.
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LT (Low Tension) and HT (High Tension) Series Reactors
LT Series Reactors (Low Tension):
- Voltage Level: LT reactors are used in low-voltage systems, typically below 1,000 volts.
- Application: LT reactors are often paired with capacitor banks in industrial and commercial settings to mitigate harmonics, limit inrush currents, and protect low-voltage equipment like motors and drives.
- Use in Power Factor Correction: They are frequently used with LT capacitor banks in power factor correctionapplications for commercial buildings and small industrial plants.
- Harmonic Filtering: Tuned or detuned reactors in low-voltage systems help reduce harmonic distortion and improve system reliability.
HT Series Reactors (High Tension):
- Voltage Level: HT reactors are designed for use in high-voltage systems, typically above 1,000 volts (ranging from 11 kV to 400 kV and beyond).
- Application: HT reactors are commonly used in power transmission and distribution networks for the following:
- Limiting fault currentin high-voltage lines.
- Stabilizing voltagefluctuations across long transmission distances.
- Protecting HT capacitor banksin large-scale power factor correction systems.
- Use in Substations: HT series reactors are also used in substations to protect high-voltage equipment, manage harmonics, and ensure safe and stable transmission of power.
- Harmonic Control: Just like in LT applications, HT reactors help manage harmonic currents in high-voltage systems, which is crucial for the stability and efficiency of large power grids.
Can We Use Capacitor Banks Without Series Reactors?
Yes, capacitor banks can be used without series reactors, but doing so may introduce several risks, especially in systems with significant harmonic distortion. Capacitor banks are primarily used for power factor correction, but without series reactors, the following issues may arise:
- Resonance:
- Capacitors have low impedance at high frequencies, which makes them more prone to resonate with the harmonic frequencies present in the electrical system.
- Resonance between the capacitor bank and the system impedance can lead to harmonic amplification, increasing the risk of damaging equipment, overheating, and failure.
- Harmonic Overloading:
- In systems with significant harmonic currents, capacitors can absorb these harmonics and become overloaded, which can lead to premature capacitor failure.
- Without series reactors to filter or detune the harmonics, the capacitor bank can face higher stress due to the presence of high-frequency currents.
- Inrush Current:
- When capacitor banks are switched on, a large inrush current occurs, potentially damaging the capacitor itself or other connected equipment. Series reactors limit these inrush currents, providing protection.
Therefore, while it is possible to use capacitor banks without series reactors, this is generally avoided in systems where harmonic distortion is significant, or where the capacitors are at risk of overloading or damage.
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Why Use 7% Detuned Reactors ?
A 7% detuned reactor is commonly used in power factor correction systems to prevent resonance with harmonic frequencies and protect the capacitor bank. The “7%” refers to the degree to which the reactor detunes the system’s resonant frequency below the harmonic frequencies, typically the 5th harmonic (250 Hz in a 50 Hz system).
Reasons for Using 7% Detuned Reactors:
- Avoiding Resonance:
- The detuned reactorshifts the resonant frequency of the capacitor-reactor combination below the 5th harmonic, which is often the most dominant harmonic in industrial systems.
- By detuning to 7%, the resonant frequency is typically shifted to around 189 Hzin a 50 Hz system, which ensures the system doesn’t resonate with the 5th harmonic (250 Hz) or other higher-order harmonics.
- Harmonic Protection:
- A 7% detuned reactor prevents the capacitor bank from absorbing high levels of harmonics, protecting the capacitors from overheating and potential failure.
- It also limits the amplification of harmonics, which can otherwise lead to poor power quality and damage to sensitive electrical equipment.
- Improved Power Factor Correction:
- While the primary purpose of detuned reactors is to avoid harmonic resonance, they still allow the capacitor bank to function effectively for power factor correction.
- The system remains safe from the risk of overloading due to harmonics while still improving power factor and reducing reactive power charges.
- Optimal for Industrial Systems:
- In many industrial setups, the 5th and 7th harmonics are the most problematic. A 7% detuned reactor offers an effective balance between filtering harmonics and improving power factor in such environments.
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Why 7% Detuning Specifically ?
The 7% detuning is chosen because it shifts the natural frequency of the capacitor-reactor circuit to approximately 189 Hz, which is below the 5th harmonic (250 Hz in a 50 Hz system). Here’s why this matters:
- Targeting the 5th Harmonic:
The 5th harmonicis the most prevalent harmonic in industrial power systems, usually generated by nonlinear loads like drives and inverters. A 7% detuned reactor shifts the resonance point below this critical frequency, thereby preventing harmonic amplification at the 5th harmonic. - Optimal Power Factor Correction:
A 7% detuning factor allows the capacitor bank to correct the power factor effectively while still protecting the system from harmonic disturbances. - Balancing Power Factor and Harmonic Protection:
Detuning at 7% is a balance between effective power factor correction and harmonic protection. It provides enough inductance to prevent resonance without significantly reducing the capacitor bank’s power factor correction capabilities.
Conclusion:
- Capacitor banks can technically be used without series reactors, but doing so can lead to problems such as harmonic amplification, inrush current, and reduced equipment life.
- 7% detuned reactorsare commonly used with capacitor banks to prevent resonance, especially with the 5th harmonic, while ensuring that the capacitor bank performs its power factor correction function efficiently.
Additional Information :
Series Reactors :
- An electrical reactor is essentially an inductor, also referred to as an inductive or magnetic coil.
- When alternating current (AC) is applied to an inductive coil, it exhibits inductive reactance. Electrical reactors work based on the same principle.
- Reactors operate exclusively with AC supply and are designed to block high-frequency signals while allowing low-frequency signals to pass through.
- Electrical reactors are engineered to have minimal internal resistance and high inductance.
- The diagram below illustrates various types of reactors in the power system.
Applications of Series Reactors:
- Series reactors are used in electrical power transmission and distribution systems to limit excessive current during abnormal conditions.
- They are employed in parallel networks to facilitate load-sharing.
- Series reactors help in balancing impedance in electrical circuits.
- They also play a role in neutral earthing applications.
- In motor starter circuits, series reactors are used to reduce high inrush currents during startup.
- During alternator parallel operations, series reactors minimize the risk of short circuits under abnormal conditions.
- They are additionally used to limit fault currents caused by harmonics or transient conditions.
Shunt Reactors:
A shunt reactor is an electrical device used in high-voltage power transmission systems to help stabilize voltage during load fluctuations.
- Shunt Reactor :
- Shunt Reactorconnected in parallel with the system or circuit.
- Main Function:
- The primary function of a shunt reactor is to allow or pass current, typically low-frequency signals or alternating current.
- Due to its characteristics, it is sometimes referred to as a current absorber.
- Operation and Design:
- Shunt reactors have high internal resistance, which typically blocks current under normal conditions, but allows current to pass during abnormal conditions.
- In high-voltage systems (400 kV or above), a three-phase shunt reactor is connected to the electrical bus to compensate for capacitive reactive power and manage dynamic over-voltage issues caused by load rejections.
- Voltage Withstanding Capability:
- A shunt reactor should be designed to withstand continuous operating voltage, typically 5% higher than its rated voltage (in a 400 kV system), without its temperature exceeding 150°C at any point.
Shunt Reactor Uses and Applications:
- Shunt reactors are used to suppress high capacitance effects in transmission and distribution systems.
- They help regulate high transient voltages in transmission and distribution networks.
- Shunt reactors act as reactive power absorbers or compensators during periods of low load.
- Similar to shunt capacitors, shunt reactors are also employed in electrical substations for voltage balancing, improving power factor, and overall voltage management.