In a power system, the voltage may vary due to changes in the load, which can affect the efficiency and stability of the system. To maintain the voltage within acceptable limits, voltage control equipment is used. This equipment can increase or decrease the voltage as necessary, depending on the load conditions. Here are some of the most commonly used methods for controlling the voltage in a power system are
- Off – Load Tap Changing Transformer
- On – Load Tap Changing transformer
- Shunt Reactors
- Synchronous Phase Modifiers
- Shunt Capacitor
- Static VAR System (SVS)
1. Off – Load Tap Changing Transformer:
Off-load tap changing transformers (OLTC) are devices used in electrical power systems to regulate the voltage levels. In this method, the transformer's turn ratio is altered to control the voltage levels. However, this is done when the transformer is disconnected from the supply, and the tap changing is done manually.
The off-load tap changer mechanism is built on the top of the transformer and consists of a moving contact that is connected to the leads of the transformer windings. The mechanism can be used to tap the transformer winding at any turn and modify the turn ratio by connecting the appropriate tap to the leads.
The voltage control through the off-load tap changing transformer is accomplished by connecting the correct tap to the transformer's winding based on the measured voltage levels. A transformer with a higher voltage output requires a higher voltage level to be fed to the transformer's primary winding, and the transformer with a lower voltage output requires a lower voltage level.
Off-load tap changing transformers are most frequently used in power systems that experience significant variations in the load current, and they are installed in the primary or secondary circuit of the transformer. By utilizing the off-load tap changer mechanism, it is possible to regulate the voltage at the transformer's secondary terminals, resulting in the voltage being regulated throughout the power system.
While this method of voltage control is effective, it requires the transformer to be disconnected from the supply before any changes can be made. Additionally, the tap-changing process is usually done manually, which can be time-consuming and may require the system to be shut down.
2. On–Load Tap Changing transformer:
An on-load tap changing transformer is a type of power transformer that can adjust the voltage of the secondary winding while the transformer is energized, that is, while it is delivering a load to the system. The on-load tap changer (OLTC) is a device that enables the tap changes to be made under load conditions without interrupting the power supply.
OLTC works by connecting a series of taps on the transformer's secondary winding to a switching mechanism, which allows the position of the taps to be changed, thus varying the turns ratio of the transformer. This variation in turns ratio changes the output voltage of the transformer. The switching mechanism is operated by a motor drive, which is controlled by a voltage regulator. The voltage regulator monitors the output voltage and signals the motor to change the tap position to maintain the desired voltage level.
On-load tap changers are used in power systems where the voltage regulation is required, and the load conditions are highly variable. The OLTC can quickly respond to changes in the load, maintaining the voltage at the desired level. The OLTCs are generally installed in high voltage transmission systems, where voltage control is critical. They are also used in distribution networks where the load conditions vary significantly throughout the day.
OLTCs offer several advantages over off-load tap changers (OLTCs). They allow for precise voltage control, which is essential in many applications. They also eliminate the need for power interruptions when changing the taps, reducing the downtime and maintenance costs. However, they are more expensive and require more complex control systems than off-load tap changers.
3. Shunt Reactors:
Shunt reactors are inductive devices that are connected in parallel with a transmission line, and they are used for controlling the reactive power flow in power systems. The primary function of a shunt reactor is to compensate for the capacitive current generated by long transmission lines or underground cables.
During normal operating conditions, the shunt reactor allows the flow of capacitive current from the transmission line to ground, which helps to balance the reactive power in the system. When the system voltage is too high, the shunt reactor absorbs some of the excess reactive power, thus reducing the voltage level. Conversely, when the system voltage is too low, the shunt reactor releases reactive power, which helps to raise the voltage level.
Shunt reactors are typically used in long-distance EHV and UHV transmission lines to regulate the voltage levels and improve the stability of the power system. They are often installed at the sending end, receiving end, or intermediate substations of the transmission line, and are usually placed at a distance of around 300 km to limit the voltage at an intermediate point.
4. Synchronous Phase Modifiers:
A synchronous phase modifier is an important device used in power systems to regulate the voltage and improve the power factor. It is essentially a synchronous motor that runs without any mechanical load and is connected to the load at the receiving end of the transmission line. The motor absorbs or generates reactive power, as needed, by varying the excitation of the field winding.
When the load requires more reactive power, the synchronous phase modifier absorbs reactive power from the system, and when the load requires less reactive power, it generates reactive power and supplies it to the system. This helps to keep the voltage constant at any condition of the load and improve the power factor of the system.
The synchronous phase modifier is particularly useful in long-distance transmission lines, where the voltage drop and reactive power demand are high. By regulating the reactive power, the synchronous phase modifier helps to reduce the voltage drop and maintain the voltage level within acceptable limits. It is often used in conjunction with other devices such as shunt capacitors, shunt reactors, and static VAR compensators to provide complete reactive power control in power systems.
5. Shunt Capacitor:
Shunt capacitors are a type of reactive power compensation equipment used in electrical power systems to improve the power factor and voltage stability. They are capacitors connected in parallel with the power line, which inject reactive volt-amperes to the system, compensating for the lagging reactive power caused by inductive loads in the system. Shunt capacitors are installed at receiving end substations, distribution substations, and switching substations, wherever reactive power compensation is required.
The shunt capacitor banks are usually composed of three-phase capacitor units, with each unit consisting of a number of individual capacitors connected in series and parallel. The capacitance of the shunt capacitor banks is selected based on the reactive power requirements of the load, and the voltage rating is selected based on the voltage level of the system.
The main function of shunt capacitors is to improve the power factor of the system by reducing the lagging reactive power caused by inductive loads. This reduces the amount of current required to deliver a given amount of real power, which leads to a reduction in line losses and improves voltage stability. Shunt capacitors also help to maintain a constant voltage level by absorbing or injecting reactive power, depending on the load conditions.
In addition to improving power factor and voltage stability, shunt capacitors also provide a number of other benefits to the power system. They help to reduce the overall size of power transformers and other equipment, reduce energy consumption, and improve the overall efficiency of the system.
6. Static VAR System (SVS)
Series Var Systems (SVS) is a type of flexible AC transmission system (FACTS) technology used to improve the stability and control of power systems. Static VAR compensator (SVC) is a key component of SVS, which is designed to regulate the reactive power in the system.
SVCs can either absorb or inject reactive power to the system, depending on the voltage level. When the voltage becomes higher than the reference value, the SVC absorbs reactive power, whereas it injects reactive power when the voltage becomes lower than the reference value. This helps maintain the voltage within the acceptable range and improves the stability of the power system.
In SVCs, thyristors are used as switching devices instead of circuit breakers. Thyristors are semiconductor devices that can switch on and off rapidly, providing fast and accurate control of reactive power. Thyristor switching is faster than mechanical switching, which enables SVCs to respond quickly to changes in the power system. Additionally, thyristor switching provides transient-free operation, reducing the risk of damage to the power system components.