We have different components in our grid substations to deliver the power safely. A power grid sub-station is composed of a bus bar, transformer, circuit breakers, isolators, lightning arrestors, capacitors bank, and different feeders. In this article, we will study the lightning arrestor and its test to check its leakage current.
What is a Lightning Arrestor?
As its name indicates, lightning arrestors protect from lightning in the power system. In case of lightning, it absorbs the high voltage value surge and diffuses it to protect the system. Here is the symbol that is represented for LA in the single-line diagram of our substation:
It provides a high resistance value at low voltage values and similarly, it helps to provide low resistance at high voltages. This is how it works to protect the system from lightning. Lightning then absorbs in earth due to its working.
Lightning arrestors are placed on both sides of the equipment which is required to be protected.
Two types of voltage surges need to be protected:
- Internal Voltage Surge
- External Voltage Surge
In external surges, lightning occurs, and because of our earthed system, it can damage the system. In case of internal surges, when we on or off the circuit breakers (switch on or switch off the power line), then there is the peak of voltage called a surge.
The leakage current is a vector sum of capacitive and resistive current through the lightning arrestor.
So both types of surges are protected with the help of lightning arrestors.
Common Lightning Arrestors
We use metal oxide lighting arresters mostly in our sub-stations and power lines, due to construction material ZnO to protect from lighting. Zinc oxide has the characteristics of passing minimum current by offering low resistance at high voltage and high resistance at low voltage. A lightning arrestor is made up of blocks of ZnO. The size and number of ZnO elements are dependent on the rating and design of specific lightning arrestors.
Here are some common types of lightning arrestors in our power system.
- ROD-type lightning arresters
- Spear-type lightning arresters
- Horn-type lightning arresters
Here is a rod-type lightning arrester:
It is conceded with a power line of specific voltage from one end through one contact. From its bottom, a wire is ousted to the lightning arrestor counter. The lightning arrestor counter has one incoming point and the second point is earthed to the grid earthing point. It will count the surge when any large voltage value comes into the system.
The second part of the lightning arrestor is earthed. It means we are connected to power lines to earth point but we complete earth through a lightning arrestor.
LCM Test Kit
There is an LCM kit present to check the healthiness of the lightning arrestor. LCM test stands for leakage current monitoring test. It is done to check the resistance value in the way of voltages we have studied its characteristics.
Here is the LCM kit:
It has a measuring unit and one part is used to measure current. The second part of this kit is fill-probe. We will keep this fill probe in to lower part of the lighting arrestor to check induction. FRP (fiber-reinforced plastic) rod is present for the filled probe as we have to go to the voltage point above near the power line.
LCM Test
We will connect the current probe to the wire of the lightning arrestor to its counter. The kit is wireless connecting with that current probe.
The fill probe contains two discs to check the fill. One disc is placed on the lower part and the second disc is placed on the above part of the rod of fill probe.
Now by inserting the fill probe into the FRP rod, we will move to the above power line.
System Harmonic Compensation in LCM Test
LCM kit will be charged and earthed first. The CT unit Measures Total Leakage Current along with total harmonics, that is system generated harmonics & and harmonics due to ZnO Blocks. It is not advised to touch the field probe unit to the base of LA (lightning arrestor), or any live part, under any circumstances as it can damage the unit.
Harmonics are generated due to the non-linear loads in the electric grid, which includes the third harmonic.
Since 3rd harmonic component is predominant in the resistive leakage current of ZnO which is roughly around 10% to 40 resistive leakage current. That’s why, we focus, mainly on calculating the third harmonics due to ZnO blocks only & and not the system harmonics, for the exact health assessment of LA.
This 3rd harmonic resistive leakage current is the main cause of deterioration of ZnO blocks and subsequently, it causes LA failure. This is why system harmonics compensation is important for better results.
Results
As a result of the LCM kit, we will get current values in small units. To decide, whether LA is healthy or not, we need to see the resistive leakage current value (Ir). This Ir value ranges from 10 microamperes to 600 microamperes, depending upon the rating, design, and age of LA.
For example, total leakage current = 755 mico Amps.
Resistive leakage current = 31 micro Amps.
Both values show the good health of the lightning arrestor.
Conclusion
A power distribution grid is composed of different power elements like lightning arrestors protected from lightning in the power system. Leakage current value will decide the lightning arrestor’s health. For that purpose, we have an LCM kit for its testing. We use metal oxide lighting arresters mostly in our substations and power lines. It offers less resistance at higher voltage and vice versa.
