Ferranti Effect

That sounds like the name of a super fancy car, Ferrari, right?

Ferranti effect (a phenomenon) occurs when the receiving end voltage (Vr) of a transmission line is greater than the sending end voltage (Vs). This phenomenon typically happens when there is low or no load on a such transmission line.

Under normal operating conditions, the sending end voltage of a line is higher than the receiving end due to voltage drop or line losses. However, for a long transmission line (> 240 km or > 150 miles) with light loading or no load, the Ferranti effect likely occurs because of a significant amount of inductance and capacitance distributed across the length of the line.

The charging current due to capacitance leads the voltage drop across the line which is in phase with the sending end voltage. The voltage drop increases along the line and adds up till the end of the transmission line, hence the receiving end voltage tends to be higher than the sending end voltage.

The increase can be written/stated as a percentage or factor.

A voltage rise at the receiving end of the circuit has a devastating consequence (depending on magnitude) if no mitigation strategy is used to control such occurrences. Equipment insulation could be damaged leading to system failures or short circuit faults. Transmission lines typically carry a bulk supply of load to final consumers and losing such lines is not a favorable operational condition in energy management, system reliability, safety, and economics wise.

It is possible to use protection functions to isolate circuits under this undesirable condition, however, re-energizing transmission lines takes a lot of effort, and due diligence, and most importantly affects many customers due to downtime.?

Some ways to mitigate Ferranti effects are:

1.??Using Flexible AC Transmission Systems (FACTS) devices to compensate or control reactive power. Thyristor-switched capacitors or thyristor-controlled reactors could control the amount of reactive power at the receiving end or various locations in the network. Static Var compensators, STATCOM, and synchronous condensers, among others, are suitable for such purposes.

2.??Shunt reactors installed at the receiving end of transmission lines (or across the tertiary winding of a power transformer) absorb or consume reactive power due to the capacitive charging current of the lines.

3.??Unified power flow controllers have the capability of fast-acting reactive power compensation.?

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