What is a Shunt Reactor?
Shunt Reactor
A shunt reactor is an electrical device used to stabilize voltage across load variations in high-voltage power transmission systems. A standard shunt reactor has a fixed rating and remains connected to the transmission line based on the load requirements.
Working Principle of Shunt Reactor
A shunt reactor absorbs reactive power in an electrical system, helping improve its efficiency. It provides reactive power compensation in high-voltage transmission lines and cable systems and may be connected directly to the power line or to the tertiary winding of a three-winding transformer. A circuit breaker is used to connect or disconnect the reactor.
Shunt reactors must withstand high continuous operating voltage. For example, in a 400 kV system, the reactor must sustain 5% above the rated voltage without exceeding 150°C at any point. Core losses may occur during normal operation; hence proper design considerations are essential to reduce these losses.
Shunt reactors are widely used in high-voltage transmission networks to regulate voltage across varying load conditions.
Construction of a Shunt Reactor
Shunt Reactor Circuit Diagram
A shunt reactor is mainly used to compensate for the capacitive reactive power of a transmission line. Its construction varies depending on the manufacturer.
Shunt Reactor Circuit Diagram
Shunt reactors resemble power transformers but typically have only one winding per phase. Their windings can be connected in a star configuration with an accessible neutral point (YN). This neutral point may be connected to the earthing system through the tertiary winding of a power transformer. They are available in two versions: conservator/dry type and oil-immersed type.
The protections and accessories are similar to those used in power transformers. In oil-immersed reactors, the oil pressure, relief valve, and air breather must be given special consideration.
Types of Shunt Reactors
1. Dry-Type Shunt Reactor
Dry-type shunt reactors are usually rated up to 34.5 kV. They are commonly connected to the tertiary winding of a transformer and used for compensated transmission lines. These reactors are air-core and can be installed indoors or outdoors. Natural convection air cooling is used, with windings spaced for airflow. Mechanical bracing, concrete, ceramic, or polyester supports are used to hold windings in place.
Dry-type reactors generate high external magnetic fields since they have no magnetic shielding. Proper clearances, arrangement, and shielding are important to avoid heating and arcing in nearby metallic structures.
Advantages: Lightweight, low cost, lower losses, and reduced maintenance.
Disadvantages: Limited to lower voltage ratings and produce strong external magnetic fields. They have no iron core and therefore no magnetizing inrush current.
2. Oil-Immersed Shunt Reactor
Oil-immersed shunt reactors come in two types: coreless and gapped iron-core.
- Coreless reactors: Include magnetic shielding to confine the flux inside the tank.
- Gapped iron-core reactors: Have higher energizing inrush currents.
These reactors are available as single-phase or three-phase units and resemble power transformers in appearance. They may use forced or self-cooling methods.
Measurement of Losses in Shunt Reactor
Shunt reactor losses are measured at rated voltage and frequency. For extremely high-voltage reactors, it may be difficult to generate the required test voltage. In such cases, losses are measured at a reduced voltage and corrected using the square of the current ratio.
Because shunt reactors have a low power factor, traditional wattmeter’s may yield inaccurate results. Instead, a bridge measurement technique is recommended for better accuracy.
The test cannot separate losses of individual components. It is advisable to perform tests when the winding temperature matches the reference temperature to avoid correction errors.
Applications of Shunt Reactors
- Improve efficiency by absorbing reactive power in transmission lines and cables
- Enhance system energy efficiency by acting as reactive power absorbers
- Used in high-voltage (HV) and extra-high-voltage (EHV) systems
- Variable shunt reactors support dynamic optimization of network conditions
- Improve voltage regulation during low-load or no-load conditions
- Enhance power quality and voltage stability