What is electrical power.

We know of active and reactive power from text books and other sources. But the basic concept to electric power and its constitutes are most of the time misleading and vague as these are some virtual terms. After studying and searching through internet I have gathered some concept on electrical power- active, reactive power, apparent power.

Electrical power:

In simple word we can define electrical power as the multiplication of voltage and current at any instant. We know P=V*I, Power= voltage*current. But this needs some extended information.

Power in Direct Current (DC) circuit

In case of Direct Current, the voltage and current level are always constant. So power = voltage*current.

Power in Alternating Current (AC) circuit

In case of Alternating current with inductive and capacitive load this electrical power is called apparent power. As in this case the electrical power is divided in to two parts active power and reactive power.

Thus with Alternating Current, Apparent Power= RMS value of Voltage * RMS value of current. [RMS- root mean square].

Blinking of AC Lighting captured by high powered camera. Light is emitted twice each cycle.

How to choose surge arrester to protect electrical equipment from traveling wave

Surge arrester is a vital protective precaution in power system. Its sole purpose is to protect the delicate electrical equipment like generator, transformer and equipments in substation. A direct fault is likely to happen rarely but any indirect fault in outside transmission line which create traveling wave will cause severe harm to the indoor or outdoor equipments.

So special care should be taken in to consideration to choose a surge arrester as per protection level and costing. After all surge arrester protects the equipments which are considered to be the heart of electrical system.

Things to consider to choose surge arrester

01. MCOV value in kV

MCOV – Maximum Continuous Operating Voltage (MCOV) provided by the manufacturer. The surge arrester’s MCOV must be greater than (or equal to) the service voltage of the network in consideration.

The choice of a surge arrester should take into consideration the voltage between the conductors and the earth (phase to neutral voltage) whereas in general the service voltage of the network is given in phase to phase voltage.

02. Voltage & Current – supplied level of protection

The effectiveness of surge arrester protection is measured by the residual voltage, at the surge arrester terminals, while a given current is flowing through it. Typically, the level of protection is defined by a pair of values (for example 80 kV/10 kA).This voltage value must be in range of withstand value of the equipment.

The margin must be at least 20% and should take the surge arrester’s installation mode and cabling into account. Ineffective cabling can lead to voltages in the equipment’s terminals that are considerably higher than the surge arrester’s residual voltage, due to voltage drops in stray impedencies.

03. Thermal withstand

It aims at guaranteeing the non-destruction of the surge arrester in the case of a long impulse (greater than the normalised test impulse). This performance is given by a current impulse withstand.

High voltage surge arrester.

surge arrester assembly

Why surge arrester is better for lightning protection against traveling wave.

Surge arrester main concept is to utilize the nonlinear behaviour with the rising of voltage level. Say for low voltage it shows mega ohm range resistance but with high rising voltage its resistance drops to 10 ohm range.

As for spark gap which is always constant for any level of voltages, surge arrester is better as it changes its value as per the rising speed of the voltage and its level.

Characteristics of surge arrester

zinc oxide (ZnO) surge arresters act like strongly non-linear resistors.

Under normal service conditions, the several Megohm withstand and the current, which remains low (in the realm of mA), limits dissipated power.

Under voltage surge conditions, the withstand greatly drops (up to roughly 5 or 10 ohms), thus limiting the voltage in surge arrester terminals and therefore in the equipment for the duration needed for the lightning current to runoff to the earth.