What is Series Compensation?
Series compensation involves inserting a capacitor or an inductor in series with a transmission line to improve its voltage transmission characteristics. By inserting reactive power in series with the transmission line, the impedance of the system is reduced, which improves the power transfer capability of the line. This is particularly useful for long transmission lines, as the voltage drop along the line can be significant, leading to reduced voltage at the load end.
Series compensation can be achieved using fixed or switched capacitors, or by using a thyristor-controlled reactor (TCR). Fixed capacitors provide a constant level of compensation, while switched capacitors can be turned on or off as needed to adjust the level of compensation. TCRs can provide continuous and adjustable series compensation by using thyristors to control the flow of current through the reactor.
In addition to improving the power transfer capability of the transmission line, series compensation can also help to reduce transmission losses and improve the stability of the transmission system. It is commonly used in extra high voltage (EHV) and ultra high voltage (UHV) transmission lines.
Advantages of Series Compensation:
- Improves voltage transmission: By injecting voltage into the transmission line, series compensation helps to improve the voltage profile at the load end and reduce the voltage drop along the line. This can help to improve the performance of equipment and reduce power losses.
- Reduces transmission losses: Series compensation can help to reduce transmission losses by improving the transmission of power over long distances.
- Improves system stability: Series compensation can help to improve the stability of the transmission system by reducing the voltage drop along the line and providing a stable source of reactive power.
- Increases power transfer capacity: By reducing the impedance of the transmission line, series compensation can increase the power transfer capacity of the line, allowing it to transmit more power.
- Easy to maintain: Series compensation devices, such as fixed capacitors, are generally self-regulating and require little or no control equipment, which makes them easy to maintain.
- Cost-effective: Series compensation is generally less expensive than shunt compensation, making it a cost-effective option for improving the transmission of power.
Disadvantages of Series Compensation:
- Limited effectiveness: Series compensation is most effective during heavy load conditions, when the voltage drop along the transmission line is significant. During light load conditions, shunt compensation may be more effective in improving the power factor of the system.
- Outage issues: When an outage occurs on a transmission line with series compensation, the series compensation must be removed to prevent overloading of the other parallel lines. This can be a complex process and may require additional protection and control measures.
- Parallel line issues: If series compensation is added to an existing transmission system, it may be necessary to have it on all lines in parallel to ensure that the system is balanced. This can be a complex and expensive process.
- High voltage issues: During system outages, the series capacitors in the transmission line may be subjected to high voltage, which can lead to damage or failure.
- Sub-synchronous resonance: Series compensation can cause sub-synchronous resonance (SSR) in some systems, which can lead to instability and damage to equipment. Additional expenses may be needed to address this issue.
What is Shunt Compensation?
Shunt compensation involves the use of a capacitor or reactor in parallel with a transmission line to improve its reactive power transmission characteristics. Shunt compensation is used to improve the power factor of the transmission system by providing a source of reactive power to the transmission line.
When an inductive load, such as a motor, is connected to a transmission line, the load current lags behind the voltage, which leads to a power factor that is less than 1. This is known as a lagging power factor. To improve the power factor, a shunt capacitor can be connected in parallel with the transmission line. The capacitor draws current that leads the voltage, which helps to improve the power factor of the system.
Advantages Shunt compensation:
- Improves voltage profile: Shunt compensation can help to improve the voltage profile of a transmission system by providing a source of reactive power to the transmission line. This can help to reduce voltage drop and improve the performance of equipment.
- Effective at all load levels: Unlike series compensation, which is most effective during heavy load conditions, shunt compensation is effective at all load levels. This makes it a useful tool for improving the power factor of a transmission system regardless of the load on the system.
- Fast control of over voltages: Shunt compensation can provide fast control of temporary overvoltages that may occur in a transmission system. This can help to protect equipment and improve the stability of the system.
- Cost-effective: Shunt compensation is generally less expensive than series compensation, making it a cost-effective option for improving the transmission of power and correcting the power factor of a system.
- Easy to maintain: Shunt compensation devices, such as fixed capacitors, are generally self-regulating and require little or no control equipment, which makes them easy to maintain.
Disadvantages of Shunt compensation:
- Higher cost: Shunt compensation is generally more expensive than series compensation, particularly when large amounts of reactive power are required.
- Limited overload capability: The overload capability of shunt compensation is limited, as the capacitors can only provide a certain amount of reactive power before they become overloaded.
- Complex control: Shunt compensation requires complex control systems to ensure that the correct amount of reactive power is provided to the transmission line. This can increase the cost and maintenance requirements of the system.
- Limited power transfer capability: Shunt compensation does not directly improve the power transfer capability of a transmission line. To increase the power transfer capacity of the line, other measures, such as series compensation or upgrading the line, may be needed.
- Risk of overvoltage: If the shunt compensation is not properly coordinated with the rest of the transmission system, it can lead to overvoltage and instability in the system.
Difference between series and shunt compensation:
- In electric power transmission, series compensation is the use of a capacitor or inductor in series with a transmission line to improve its voltage transmission characteristics. Series compensation is used to reduce transmission losses and improve the transmission of power over long distances.
- Shunt compensation, on the other hand, is the use of a capacitor or reactor in parallel with a transmission line to improve its reactive power transmission characteristics. Shunt compensation is used to improve the transmission of reactive power and to correct the power factor of the transmission system.
- One of the main differences between series and shunt compensation is the location of the compensation device in the transmission system. Series compensation is placed in series with the transmission line, while shunt compensation is placed in parallel with the transmission line.
- Another difference between the two is the type of compensation provided. Series compensation helps to improve the transmission of active power by injecting voltage into the transmission line to compensate for voltage drop. Shunt compensation, on the other hand, helps to improve the transmission of reactive power by providing a source of reactive power to the transmission line.
- In general, series compensation is used for long transmission lines to improve the voltage profile at the load end, while shunt compensation is used to provide reactive power compensation and improve the power factor of the transmission system. The capacity of the shunt compensation device will depend on the reactive power requirement of the transmission system and the size of the lagging load.