Lightning Arrester

 

Lightning.....! the wicked thing in our power system.

                    For us it creates only damages. So is important to protect the crores worth devices which support the generation, transmission and distribution of power.

Starting from the generating stations, all the power cables are connected to a device which is called surge diverter or lightning arrester. In transmission towers we draw a separate line for earth at the top of the tower. By that line we create a zero potential at the top of tower. Thus protecting transmission lines from lightning surges.

Electrical surge produces a large transient over voltage in the electrical network and system. The shape of the transient over voltage has a steeply rising front with slowly decaying tail. There are two types of surges one comes externally from atmosphere such as atmospheric lightning. Second type is originated from electrical system itself, such as switching surges. When an electrically charged cloud comes nearby an electrical transmission line, the cloud induces electrical charges in the line. When the charged cloud is suddenly discharged, through lightning, the induced charge in the transmission line is no longer confined static. It starts travelling and originate dynamic transient over voltage. This transient over voltage travels towards both load and source side, on the transmission line because of distributed line inductance and stray capacitance. This surge voltage travels with speed of light. At the end of the transmission line, as the surge impedance changes, the surge voltage wave reflected back. This forward and backward travelling of surge voltage wave continues until the energy of the surge or impulse is attenuated by line resistance. This phenomenon causes voltage stress on the transmission system many times greater than normal rated voltage of the system. Hence, surge protection scheme must be provided to the electrical power transmission system to make reliable and healthy system.

Normal Surge Voltages and it's average magnitude(volts)

The following are the damages that are caused by the travelling wave on the substation equipment.

• The high peak voltage of the surge may cause flash-over in the internal winding thereby spoil the winding insulation.
• The steep wave fronts of the surges may cause external flash-over between the terminal of the transformer.
• The highest peak voltage of the surge may cause external flash-over, between the terminal of the electrical equipment which may result in damage to the insulator.

An ideal lightning arrester should have the following characteristics;

• It should not draw any current during normal operating condition, i.e., it sparks-over voltage must be above the normal or abnormal power frequency that may occur in the system.
• Any transient abnormal voltage above the breakdown value must cause it to break down as quickly as possible so that it may provide a conducting path to ground.
• When the breakdown has taken place, it should be capable of carrying the resulting discharge current without getting damaged itself and without the voltage across it exceeding the breakdown value.
• The power frequency current following the breakdown must be interrupted as soon as the transient voltage has fallen below the breakdown value.

In modern era, gap less ZnO or zinc oxide surge arresters are mainly used for surge protection. Let us discuss zinc oxide type gap less arresters. This type of arrester comprises of numbers of solid zinc oxide disc. This discs are arranged one by one to form a cylindrical stack. The number of zinc oxide discs used per lightning arrester depends upon the voltage rating of the system. This stack is kept inside a cylindrical housing of polymer or porcelain. Then the stack is placed inside the housing and highly pressed by heavy spring load attached to end cap at top. The equipment connection terminal for line is projected from top cap and connection terminal for earth is projected from the bottom cap.

ZnO lightning arrester construction

When system voltage is applied on the surge arrester at continuous operating voltage, about 80% of the rated voltage, it experiences some leakage current(ir). The amplitude of the leakage current depends on the condition of the surge arrester. The leakage current consists of the capacitive and the resistive current component. This leakage current generates heat. This generated heat should be dissipated properly otherwise the temperature of the lightning arrester may rise which further increases the leakage current. Because of this the proper thermal design of surge arrester housing plays an important role.

When a high voltage surge is impressed on the lightening arrester, the resistance of the ZnO becomes low and as can be seen from the voltage-current characteristics, the current through the lightning arrester becomes high of the range of kilo ampere (kA). The voltage, beyond which the current through the lightning arrester becomes such high, is known as reference voltage and the current at reference voltage is known as reference current. As lightning arrester is connected in between phase and ground, the high voltage surge is discharged to ground by low resistance offered by the Zinc Oxide blocks and in this way prevents the equipment from damage due to voltage surge.

Non-linear Resistance Characteristics of ZnO block

Another device installed along with this arrester is a counter with counts how many times the surge occurred in the power lines and also show the aptness of the arrester.

It carries a milliammeter which measures the flow of current through the surge arrester. If pointer deflects within the green range, then we could say that lightning arrester is in good condition. When it enters to the red zone, it indicates that the device is failed and immediately swap it with a good one for continuous protection for the power system. Some will have a yellow zone which convey us the fact that this arrester is depleting it's properties and so exchange is needed with a good one.