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Electric power is the rate at which electric energy is transferred by an electric circuit. It is usually transformed to other forms of power when electric charges move through an electric potential difference which occurs in electrical components in electrical circuits.
In AC circuits, the electrical components which are inductors and capacitors might go under a periodic change in the direction of energy flow. This in turn gives rise to active & reactive power.

The portion of power that averaged over a complete cycle of the AC waveform results in net transfer of energy in one direction is known as active power (real power). The portion of power due to stored energy, which returns to the source in each cycle is known as reactive power.

When we pass an electric current through a wire, it obviously produces magnetic field around it. When this field alternates between opposite peak values both in time and space, an induced voltage is produced in any of the conductors lying in the path of this field. This particular field can also react with any other magnetic field established by any other conductor and a mechanical force is created between the two conductors. This alternating field produced by an alternating current is the basis for enabling us to use electric power extensively.

A DC current with a steady, non alternating field, does not offer this advantage.

The alternating flux posed a lot of problems and one of them being the reactive power. It is the power required to establish & maintain an AC fluctuating magnetic flux without which no energy transfer can take place. Against this we have active power which delivers power in an electrical, mechanical, thermal or any other form.
Reactive power is a necessity. Without reactive power the system won’t function properly yet it is the one posing a major problem. The problem area linked with the AC fields are reactances, arcs, surges, resonances, skin effect, hunting torque. To overcome all those problems, capacitor is an effective tool.

When a voltage is initially placed across the coil, a magnetic field builds up and it takes a period of time for the current to reach full value. This causes the current to lag behind the voltage in phase. Hence, these devices are said to be sources of lagging reactive power.
A capacitor is an AC device that stores energy in the form of an electric field. When current is driven through the capacitor, it takes a period of time for a charge to build up to produce the full voltage difference. On an AC network, the voltage across a capacitor is constantly changing – the capacitor will oppose this change causing the voltage to lag behind the current. In other words, the current leads the voltage in phase. Hence, these devices are said to be sources of leading reactive power.

### Purpose of Reactive Power

1. Synchronous generators, SVC and various types of other Distributed energy resource (DER) equipment are used to maintain voltages throughout the transmission system. Injecting reactive power into the system raises voltages and absorbing reactive power lowers voltages.
2. Voltage-support requirements are a function of the locations, magnitudes of generator outputs, customer loads and of the configuration of the DER transmission system. These requirements can differ substantially from location to location and can change rapidly as the location and magnitude of generation and load change. At very low levels of system load, transmission lines act as capacitors and increase voltages. At high levels of load, however transmission lines absorb reactive power and thereby lower voltages. Most transmission system equipment (e.g. capacitors, inductors, and tap-changing transformers) is static but can be switched to respond to changes in voltage-support requirements.

System operation has three objectives when managing reactive power and voltages.

1. It must maintain adequate voltages throughout the transmission and distribution system for both current and contingency conditions.
2. It seeks to minimize congestion of real power flows.
3. It seeks to minimize real power losses.

Here are five of the most common factors that affect power transmission lines:

1. Line Losses: Naturally, the transfer of electrical energy between power plants, substations and customers is impossible without some energy loss. The gap is referred to as the line loss.

2. Transmission Rates: Transmission rates encompass the cost of providing transmission service and reflect each individual transmission owner’s investment in the transmission infrastructure to yield a return.

3. Voltage Fluctuation: Voltage fluctuations are changes or swings in the steady-state voltage above or below the designated input range for a piece of equipment. Fluctuations include both sags and swells.

4Transients: A transient is a high voltage spike caused by external or internal transient sources. A transient is a high voltage spike of less than 10 microseconds in duration.

5. Over Voltage: A voltage greater than that at which a device or circuit is designed to operate also known as over potential. The amount by which the applied voltage exceeds the Geiger threshold in a radiation counter tube.