Requirement of Capacitor Banks
Let us first discuss the actual requirement of capacitor bank in the power system. Actually, inductive loads such as transformers, motors, reactors, transmission networks, etc., cause current to lag behind the voltage. This increases the phase angle between current and voltage which in turn reduces the cosine of that angle. The cosine of phase angle is known as power factor. That means it makes the power factor poor.
Now a capacitor causes the current to lead the applied voltage. So, when a capacitor bank is installed in the system, the leading current counteracts the lagging current, hence the load current will appear closely in phase with voltage. That means power factor is improved. Because of improved power factor, the current of the power is reduced.
Therefore, we can say that the use of capacitor bank reduces the system current. As the current is reduced, the voltage drop in the system decreases. Because the expression of voltage drop is given as. \[ \Delta V = I(X\sin\theta + R\cos\theta ) \]
So decreased current improves voltage regulation. Therefore, the system voltage is elevated from its lower value even without changing the tap position of transformers.
Again, reduction in current means reduction in I²R or ohmic loss in the system, and therefore the overall system loss gets reduced. A capacitor bank also reduces the ampere loading of the system.
When we connect a capacitor bank, the current required to transmit a given amount of power decreases. This reduced current allows the system to deliver more active power without exceeding its current carrying capacity. That means, a capacitor bank enables more active power to transmit by the same network. Therefore, the expenditure behind infer structures for each unit of active power is reduced.
Types of Capacitor Bank
There are mainly two types of capacitor bank used for high voltage system. The shunt capacitor banks and series capacitor banks.
Shunt Capacitor Bank
Shunt capacitor bank is connected across the system. A shunt capacitor bank improves power factor by reducing the phase angle between the current and voltage. As the angle between voltage and current is reduced, the reactive power that is (\(VI \sin\theta \)) also gets reduced. A shunt capacitor mainly reduces the current with increasing \(\cos\theta \)) for same active power (\(VI \cos\theta \)). The reduction in current reduces the voltage drop in the system thereby improves voltage regulation. It also reduces the \(I^2R\) loss i.e. ohmic loss in the system by reducing system current. Because of reduced current and increased power factor \((\cos\theta \)) it improves the capacity of power transmission system with the same current carrying capacity (or ampacity).
Series Capacitor Bank
Now let’s discuss the series capacitor bank. It is connected in series with the transmission line. Since the entire load current flows through it, a series capacitor cannot reduce the system current directly. Actually, as it is connected in series with the line, it reduces the inductive reactance of the power line. Due to inductive loads, the system gets some inductive reactance. Also, a power line itself has its inherent inductive reactance. Connecting a series capacitor bank to the line implements capacitive reactance to the line. That capacitive reactance counteracts the inductive reactance and thereby overall reactance of the line gets reduced. The expression of voltage drop of a transmission line is given by \[ \Delta V = I(X\sin\theta + R\cos\theta ) \]
Since a series capacitor bank reduces the overall reactance of the transmission line, it reduces the voltage drop in the line. This leads to improved voltage regulation of the transmission system. It is evident that the improvement in voltage regulation is primarily due to the reduction in reactance, and not dependent on the load current.
Since the entire line current flows through the series capacitor, any fault in the system results in a large fault current passing through the series capacitor bank. As a result, the voltage across the capacitor can rise to 15 times or more than its rated value.
This is why a special protection scheme must be implemented for series capacitor banks to safeguard them against extreme over voltages during faults. Due to such challenges, the use of series capacitor banks is generally limited to extra high voltage (EHV) transmission lines.